Magnetic recording medium and method for production thereof

ABSTRACT

A magnetic recording medium having a magnetic metal thin film as a magnetic layer is disclosed. An oxide layer is formed on the surface of the magnetic metal thin film, and a proactive layer is further formed on the oxide layer. The oxide layer on the surface of the magnetic metal thin film has a thickness of 20 to 230 Å, while the protective layer has a thickness of 20 to 230 Å. The combined total thickness of the oxide layer and the protective layer is 40 to 250 Å. The oxide layer surface may be bombarded prior to formation of the protective layer for improving the bonding of the protective layer. A lubricant may be coated on the surface of the protective layer. The lubricant may be one of perfluoro alkyl ester of carboxylic acid, ester compounds of perfluoro polyether having carboxylic groups, and amine salt compounds. The reverse surface of a non-magnetic base film as a substrate has a centerline mean roughness R a  and the maximum height R max  of projections in the ranges of 0.015 μm≦R a  0.0070 μm and 0.015≦R max  ≦0.070 μm, respectively.

TECHNICAL FIELD

This invention relates to a magnetic recording medium, above all, anevaporated tape on which a thin metal film is deposited by evaporation,and a method for producing the recording medium.

BACKGROUND ART

A magnetic recording medium of a so-called magnetic metal thin filmtype, in which a magnetic material of metal or an alloy such as Co--Niis directly deposited by plating or a vacuum thin film forming technique(e.g. vacuum deposition, sputtering or 1on plating) on a base film, suchas a polyester film or a polyimide film, has a number of advantages,that is, it is superior in coercivity, rectangular ratio or inelectromagnetic transducing characteristics for a short wavelengthrange, while the magnetic layer can be reduced in thickness for reducingdemagnetization due to recording or thickness losses during playback. Inaddition, the packing density of the magnetic material may be increasedbecause there is no necessity of mixing a non-magnetic binder into themagnetic layer.

In the magnetic metal thin film type recording medium, the magneticlayer is generally formed by vacuum deposition. For example, themagnetic layer is deposited by a so-called continuous take-up typeoblique evaporation method in which an evaporated magnetic material isdeposited on the base film which is caused to run in a vacuum chamberfrom a supply side to a take-up side so as to travel on the outerperipheral surface of a cooling can provided on the travel path of thebase film.

Meanwhile, when the magnetic metal thin film type magnetic recordingmedium is employed in, above all, a digital video tape recorder, sincethe data transfer rate is extremely high, the relative velocity betweenthe magnetic recording medium and the magnetic head during recording andplayback needs to be at least twice the recording velocity forconventional analog recording. Since a considerable damage is done inthis manner to the magnetic recording medium, it has become crucial toimprove durability of the magnetic recording medium.

However, if oxygen be introduced during vacuum deposition for providinga protective layer, such as a Co oxide, on the surface of the magneticlayer, sufficient durability of the magnetic recording medium cannot beachieved. For this reason, it becomes additionally necessary to providea protective film of an abrasion-resistant material. Besides, if simplythe protective film is increased in thickness, there is a risk thatelectro-magnetic transducing characteristics be deteriorated due tospacing losses.

In view of the above-described status of the art, at is an object of thepresent invention to provide a magnetic recording medium which isimproved in abrasion resistance without incurring deterioration of theelectromagnetic transducing characteristics and which is superior indurability and reliability, and a method for producing such magneticrecording medium.

DISCLOSURE OF THE INVENTION

In its one aspect, the present invention provides magnetic recordingmedium in which, as shown in an enlarged schematic cross-sectional viewof FIG. 1, a metal magnetic thin film 102, a layer 103 of an oxide ofthe metal magnetic thin film 102 and a protective layer 104 aresequentially formed on a substrate (base film) 101, the oxide layer 103and the protective layer 104 having thicknesses t₁ and t₂ of 20 to 230 Åand 20 to 230 Å, respectively, with the combined thickness T of theoxide layer 103 and the protective layer 104 being 40 to 250 Å.

In another aspect of the present invention, the protective layer 4 ofthe magnetic recording medium is formed of an inorganic material.

During deposition of the protective layer, the surface of the oxidelayer may be previously bombarded for improving contact tightness of theprotective film.

That is, after the magnetic metal thin film is formed on the base filmwithin a vacuum chamber by vacuum deposition, the as-deposited magneticmetal thin film may be bombarded at a making power density of 1.6 kW/m²or higher, and a protective film may then be formed continuously on thebombarded magnetic metal thin film within the same vacuum chamber. Thisis the second subject-matter of the present invention.

The magnetic metal thin film, which is to be a magnetic layer, is formedby a vacuum deposition method. The vacuum deposition method includes notonly the usual vacuum deposition method, but also a method in whichevaporated chemical species are ionized and accelerated under the actionof an electrical field, magnetic field or electron beam irradiation forallowing the magnetic thin film to be deposited on the base film in anatmosphere providing a longer mean free path for the evaporated chemicalspecies.

For depositing the magnetic metal thin film by the above-describedvacuum deposition method, the so-called oblique vacuum deposition methodis utilized in which a magnetic material evaporated from a source of thematerial is incident and deposited from a predetermined inclinedincident direction on the surface of a non-magnetic base materialtravelling along the outer periphery of a cooling can which is adaptedfor being rotated in a predetermined rotational direction. An oxygen gasis introduced onto the surface of the non-magnetic base material forimproving durability and magnetic properties of the magnetic metal thinfilm.

After deposition of the magnetic metal thin film by the vacuumdeposition method, the above-mentioned bombardment operation is carriedout. This assures a sufficient bonding power between the magnetic metalthin film and the protective film for improving durability andreliability even if the protective film is deposited on the magneticmetal thin film. On the other hand, spacing losses may be minimized andthe electromagnetic transducing characteristics may be improved.

The gas used for bombardment is preferably an inert gas, specifically anAr gas, for avoiding oxidation of the surface of the magnetic metal thinfilm.

For bombardment, the making power density at a pair of electrodesprovided within a bombardment device is to be 1.6 kW/m² or higher. Ifthe making power density is lower than 1.6 kW/m², sufficient etchingeffects on the film surface cannot be expected such that the bondingpower of the protective film cannot be improved. The making powerdensity represents the processing capacity per unit area. It isdesirable to set the voltage and current applied to the electrodes independence upon the tape speed or tape width of the base film in orderfor the making power density to be within the above-mentioned range.

After the above-mentioned bombardment, the protective film is depositedon the bombarded surface within the same vacuum chamber. This assures asufficient bonding power of the protective film to prevent theprotective film from becoming delaminated during sliding contact of themagnetic head to improve durability and reliability.

There is no particular limitation to the method of film deposition andany of sputtering, CVD and vacuum deposition may be employed. However,such a method which permits in-line processing and a magnetic metal thinfilm depositing process is most preferred.

A carbon film is generally preferred for the protective film. However,any of an SiO₂, Si₃ N₄, SiN_(x), BN, ZnO₂, Al₂ O₃, MoS₂ or SiC film,which is formed by sputtering, may also be employed.

Also, any of customarily employed magnetic materials may be employed asa material constituting the magnetic metal thin film. However, amagnetic metal material is preferred. Any of magnetic metal materialscommonly employed with this type of the magnetic recording medium may beemployed. Examples of these materials include magnetic metal materials,such as Fe, Co or Ni, Fe--Co, Co--Ni, Fe--Co--Ni, Fe--Co--Cr, Co--Ni--Crand Fe--Co--Ni--Cr.

The magnetic metal thin film is deposited by a vacuum thin film formingtechnique. Examples of the vacuum thin film forming technique includevacuum deposition, sputtering and ion plating. Of these, vacuumdeposition is most preferred. Besides the usual vacuum depositionprocess, vacuum deposition may be carried out in such a manner that avaporized stream is ionized and accelerated under the effects of anelectrical field or a magnetic field or by electron beam radiation forestablishing an atmosphere assuring an increased mean free path forchemical species being evaporated for allowing deposition of a thin filmon a non-magnetic base material. Typically, an oblique deposition isachieved by utilizing the vacuum thin film forming technique. By theoblique deposition is meant a method of introducing a vaporized streamof the magnetic metal material in a direction of forming a predeterminedincident angle with respect to a normal line drawn to the non-magneticbase material for allowing precipitation of the magnetic thin film onthe non-magnetic base material.

The magnetic metal thin film may be formed as a single layer or as amulti-layer. In the latter case, the magnetic metal thin film may beconstituted by two or more stacked layers of the magnetic metal thinfilms and a non-magnetic intermediate layer interposed between thesethin films. The sole non-magnetic intermediate layer is provided ifthere are two of the magnetic metal thin films, while (n-1) non-magneticintermediate layers are provided if there are n magnetic metal thinfilms, n being an integer of 3 or more. The non-metallic intermediatelayer(s) are effective to prevent magnetic interaction between themagnetic metal thin films for assuring a low noise.

The non-magnetic intermediate layer is formed of an oxide, such asoxides of Cr, Si, Al, Mn, Bi, Ti, Sn, Pb, In, Zn or Cu, or compositeoxides thereof. The combined thickness of the non-magnetic intermediatelayers is to be not more than 20% of the total thickness of therecording layer. If the combined thickness exceeds 20% of the totalthickness, it becomes difficult to improve electromagnetic transducingcharacteristics especially for the short wavelength range. Besides, thethickness of each layer is preferably not more than 300 Å. If thethickness exceeds 300 Å, it tends to be difficult to detect recordingsignals from the magnetic metal thin film underlying the non-magneticintermediate layer.

The intermediate oxide layer interposed between the magnetic metal thinfilms leads to a weakened magnetic bondage between the magnetic thinfilms. However, in case of an excessive film thickness of the oxidelayer, the energy product tends to be lowered to deteriorateelectro-magnetic transducing characteristics.

On the other hand, an oxide layer formed on the surface of the magneticlayer constituted by plural magnetic metal thin films, that is on thesurface of the uppermost magnetic thin film, is effective to improvedurability of the magnetic recording medium. However, the spacing lossestend to be produced if the oxide layer is increased in thickness.Therefore, it is crucial with the magnetic recording medium having suchmulti-layer structure to maintain a balance between durability andelectro-magnetic characteristics.

Thus, in stacking the magnetic metal thin films, the surface of theunderlying magnetic metal thin film(s) may be bombarded with an inertgas containing reducing gases.

If the surface of the magnetic metal thin film is bombarded with theinert gas containing reducing gases during vapor deposition, it becomespossible to eliminate or reduce the thickness of the intermediate oxidelayer(s).

In effect, an extremely thin thickness of the intermediate oxidelayer(s) on the order of tens of ≈ suffices. For example, if the oxidelayer formed to a thickness of the order of 100 Å during vapordeposition is reduced in thickness to the order of 20 Å by bombardment,the residual magnetic flux density B_(r) of the magnetic layer may beimproved, while the electro-magnetic conversion characteristics may alsobe improved because the magnetic flux emanated from the lower magneticthin film(s) may be prevented from being lowered.

Although there is no particular limitation to the inert gas employed forbombardment, an Ar gas, for example, is commonly employed.

The reducing gas(es) introduced into the inert gas may for example be H₂gases or acetylene.

The conditions for bombardment may be represented using a constant K asdefined by the following equation (1): ##EQU1## where E and I denote thevoltage applied to the electrodes in the processing unit and the currentthrough the electrodes, respectively. Besides, v and w in the formula(1) denote the tape speed through the processing unit and the processingwidth of the magnetic tape during bombardment.

Thus the above constant K may be thought of as denoting the processingcapability per unit area. According to the present invention, thevoltage V and the current I of the electrodes are preferably selected sothat the value of K of not less than 10 is achieved. If the value of Kis controlled to be within the above-mentioned range, it becomespossible to maintain durability and to improve electro-magnetictransducing characteristics of the magnetic recording medium.

Meanwhile, with the present magnetic recording medium, the magneticlayer may be of a dual-film structure or a multi-film structure composedof three or more magnetic thin films. In any case, the magnetic thinfilms making up the magnetic layer may be deposited so that thedirection of growth is in the same direction or forward direction or inthe opposite direction or reverse direction.

As the base film, any of those films customarily used with this type ofthe magnetic recording medium may be employed. Specific examples ofthese films include films formed of polyesters, such as polyethyleneterephthalate or polyethylene-2, 6-naphthalate, aromatic polyamide filmsor polyimide resin films.

Meanwhile, when forming a multi-layered magnetic layer as describedabove, since the magnetic metal thin film of a reduced film thickness isto be formed by increasing the travelling speed of a flexible basematerial (base film) along the outer peripheral surface of the coolingcan, the contact time between the travelling surface of the base filmand the outer peripheral surface of the cooling can is decreased so thatsufficient cooling of the base film is not achieved on the cooling canto produce the problem of heat deterioration. As a result of our eagersearches, the present inventors have found that the above inconveniencemay be overcome by controlling the roughness of the non-vapor-depositionsurface, that is the travelling surface, of the flexible base film, forcontrolling the contact area between the traveling surface of theflexible base film and the outer peripheral surface of the cooling can.

Consequently, according to the third aspect of the present invention,there is provided a magnetic recording medium having plural magneticmetal thin films deposited on one of the surfaces of the flexible basefilm, in which the roughness of the other surface, that is thetravelling surface, of the flexible base film, is defined so that themean roughness of the centerline R_(a) and the maximum height R_(max) ofthe protrusions thereof are in the ranges of from 0.0015≦R_(a) ≦0.0070μm and 0.015≦R_(max) ≦0.070 μm, respectively, for preventing thermaldeterioration and decrease in product yield and improving theelectro-magnetic transducing characteristics of the magnetic recordingmedium. Meanwhile, the definitions of the mean roughness of thecenterline R_(a) and the maximum height of the protrusions R_(max) aregiven in JIS B0601.

The surface of the flexible base film, on which the magnetic layer isformed, exhibits satisfactory magnetic characteristics and surfaceroughness sufficient to inhibit spacing losses, whereas the oppositesurface (travelling surface) thereof assures travelling characteristicsand exhibits sufficient surface roughnesses to achieve a contact areawith the cooling can enough to achieve the cooling of the flexible basefilm.

According to the present invention, the roughness of the travellingsurface is defined in terms of the mean centerline roughness R_(a) andmaximum height of the projection R_(max) such that 0.0015≦R_(a) ≦0.0070μm, preferably R_(a) ≦0.004 μm, and 0.015≦R_(max) ≦0.070 μm.

If the centerline mean roughness R_(a) is less than the above prescribedvalue, the travelling speed of the flexible base film on the outerperipheral surface of the cooling can cannot be increased sufficientlybecause of the tight contact between the travelling surface and theouter peripheral surface of the cooling can. On the other hand, theflexible base film tends to be creased to render the production of aproduct difficult. If the centerline mean roughness R_(a) is larger thanthe above prescribed value, the flexible base film travelling on thecooling can cannot be cooled sufficiently because of the excessivelyreduced contact area between the travelling surface of the base film andthe outer peripheral surface of the cooling can. The result is thedeformation of the flexible base film, such as widthwise contraction orlongitudinal elongation of the base film, under the thermal effects(thermal deterioration) and lowered electro-magnetic transducingcharacteristics. In extreme cases, there are produced streaks in thelongitudinal direction also render the product unusable 1so lower theproduction yield.

By defining the range of the maximum height R_(max) of the protrusions,it becomes possible to average the size of the protrusions of thetravelling surface. For example, if the centerline mean roughness R_(a)is within a prescribed range, and the maximum height R_(max) of theprotrusions is lesser than the prescribed range, suitable roughnesscannot be accorded to the travelling surface of the flexible base film,such that the travelling speed cannot be increased sufficiently. On theother hand, if the centerline mean roughness R_(a) is within aprescribed range, and the maximum height R_(max) of the protrusions islarger than the prescribed range, the protrusions are distributed on thetravelling surface of the flexible base film, such that not only theflexible base film travelling on the cooling can may not be cooledsufficiently, but also the running performance of the recording mediumis lowered.

The roughness of the travelling surface of the flexible base film may becontrolled by adding a filler into the base film material. The filler iscomposed of extremely fine particles which are thought to be coalescedon addition into the base film material. Therefore, it suffices if thefiller having a predetermined particle size capable of forming theprotrusions within the above-prescribed size range is added into thebase film material. There is no particular limitation to the types ofthe filler if it is such a filler as is commonly employed for this typeof the magnetic recording medium.

Besides, in the present invention, the step of forming an undercoatlayer, a back-coat layer or a top coat layer on the base film may beadded, if so desired. There is no particular limitation to theconditions of forming the undercoat layer, back-coat layer or the topcoat layer if the forming steps are those commonly employed for thistype of the magnetic recording medium. Meanwhile, the undercoat layer,back-coat layer or the top coat is preferably formed in an in-linerelation with respect to the step of depositing the magnetic layer andthe protective layer because the productivity may be improved in thismanner significantly.

The top coat layer may be formed by applying one of a variety ofcustomary lubricants. Most preferred of the lubricants are an ester ofperfluoroalkyl carboxylic acid, an ester of a perfluoro-polyether havinga terminal carboxyl group and a long-chain alcohol, and amine salts ofperfluoro-polyethers having terminal carboxylic groups.

The ester of a perfluoroalkyl carboxylic acid is a compound representedby the general formula RCOO(CH)_(j) C_(k) F_(2k+1), where R is ahydrocarbon residue, j≧0 and k>4. The hydrocarbon residue R ofcarboxylic acid may be straight-chained or branched, saturated orunsaturated, and may, for example, be an aryl group or a perfluorohydrocarbon residue.

The perfluoroalkyl carboxylic acid ester may be easily synthesized by areaction of an acid chloride and a fluorine-containing alcohol.

The acid chloride may easily be synthesized by chlorinating commercialaliphatic carboxylic acid with phosphorus pentachloride PCl₅ or thionylchloride SOCl₂. If aliphatic carboxylic acid has a smaller number ofcarbon atoms, the acid chloride may be synthesized by chlorination withthionyl chloride SOCl₂.

Fluorine-containing alcohol C_(k) F_(2k+1) (CH₂)_(j) OH may easily besynthesized by chlorination of perfluoro carboxylic acid, produced bye.g. the Simmonds method, in the presence of dimethyl formamide (DMF)followed by reduction with a reducing agent. Alternatively,commercialized perfluoro alcohol represented by the general formulaC_(k) F_(2k+1) CH₂ CH₂ OH may be employed.

For bonding the lubricant layer containing the ester of perfluoroalkylcarboxylic acid to the magnetic metal thin film, a solution obtained bydissolving the lubricant in a solvent may be coated or sprayed on thesurface of the magnetic metal thin film. Alternatively, the magneticmetal thin film may be immersed in the solution.

The coating quantity is preferably in a range of from 0.5 mg/m² to 100mg/m² and more preferably 1 mg/m² to 20 mg/m². If the coating quantityis too small, the desirable effects of low frictional coefficients andimproved abrasion resistance or durability are not displayed.Conversely, if the quantity is excessive, a phenomenon of agglutinationtends to be incurred between the sliding member and the magnetic metalthin film to lower the running characteristics.

The ester compounds of perfluoro polyether having terminal carboxylicacid and long-chain alcohol are represented by the formulas: R_(f) COORand RCOOCR_(f) COOR, where R_(f) and R are a perfluoro polyether groupand a long-chain hydrocarbon, respectively.

In addition to the ester compounds, ester phosphate or ester phosphiterepresented by the following formulas may be contained in thelubricants: (R₁ O)_(n) P(OH)_(3-n), (R₁ O)_(n) PO(OH)_(3-n), (R₁ S)_(n)P(OH)_(3-n) or (R₁ S)_(n) PO(OH)_(3-n), where R₁ is a hydrocarbonresidue.

Alternatively, the long-chain alkyl amine represented by the generalformula: R₂ --NH₂, where R₂ in a long-chain hydrocarbon, may becontained in the lubricant in addition to the above-mentioned estercompound.

It is noted that the alkyl amine is to be added in an amount of 0.01 to100, in terms of a molar ratio, with respect to the ester compound.

By the above-mentioned ester compound, coated on the surface of thethin-film magnetic recording medium, the magnetic recording medium maybe of sufficient durability even when employed under hostilelow-temperature low-humidity environments. Besides, the magneticrecording medium is not lowered in characteristics. In addition,Freon-based solvents may be eliminated by employing the long chainhydrocarbon-perfluoro polyether ester compounds.

These ester compounds may easily be synthesized by reacting perfluoropolyether having terminal carboxylic groups and the long-chain alcoholin anhydrous toluene under heating and refluxing using a minor amount ofp-toluene sulfonic acid and concentrated sulfuric acid as catalystsunder elimination of water. After completion of the reaction and removalof toluene, the resulting product is purified in a column.

The long-chain alkyl groups, as the ester compounds between perfluoropolyether having a terminal carboxylic group and the long-chain alcohol,may be selected without regard to the molecular weight, presence ornon-presence of branched structure, unsaturated structure, isomericstructure, or alicyclic structure. The number of carbon atoms ispreferably 6 or more in view of solubility. Table 1 shows the structureof the long-chain alkyl group. The structure of perfluoro polyether,having terminal carboxylic groups, is shown below only by way ofillustration.

The substituents as phosphates or phosphites may also be selectedwithout regard to the molecular weight, number of carbon atoms, presenceor non-presence of branched structure, unsaturated structure, aromaticrings, isomers or alicyclic structure. The number of the estersubstituents may be 1 to 3, whichever is possible. Although there is noparticular limitation, the amount of addition of phosphates andphosphites is preferably 30 to 70 wt % of the ester compounds.

The alkyl groups as long-chained alkyl amines may similarly be selectedwithout regard to the molecular weight, number of carbon atoms, branchedstructure, unsaturated structure, presence or non-presence of aromaticrings, isomers or alicyclic structure. The straight-chained hydrocarbonshaving 10 or more carbon atoms are preferred in view of the frictionalcoefficients. The amount of addition of the long-chain alkyl amine ispreferably 0.01 to 10, in terms of molar ratio, with respect to theester compounds.

A monofunctional perfluoro polyether may be exemplified by F(CF₂ CF₂ CF₂O)_(n) CF₂ CF₂ COOH, CF₃ [OCF(CF₃)(CF₂ ]_(m) (OCF₂)COOH, while apolyfunctional perfluoro polyether may be exemplified by HOOCCF₂ (OC₂F₄)_(p) (OCF₂)_(p) OCF₂ COOH.

In the above structural formulas of the perfluoro polyethers, l, m and neach indicate an integer of 1 or more. The molecular weight of perfluoropolyether, to which no limitation is imposed, preferably 600 to 5,000.If the molecular weight is excessive, the effect of the terminal groupsis diminished, while the perfluoro polyether fraction is increased, sothat the consumption of Freon is increased. Conversely, if the molecularweight is too low, the effect of the perfluoro polyether groups isdecreased. As for alcohol, the number of carbon atoms of at least one ofthe alkyl groups thereof is preferably 6 or more.

In order for the ester compound of the perfluoro polyether having theterminal carboxylic group and the long-chain alcohol to be retained bythe metal thin film type magnetic recording medium, it may becontemplated to apply the lubricant layer as a top coat on the surfaceof the magnetic metal thin film or on the surface of the protective filmas in the case of the perfluoroalkyl carboxylic acid ester describedpreviously. The coating quantity of the ester compound of the perfluoropolyether having the terminal carboxylic group and the long-chainalcohol is preferably 0.5 to 100 mg/m² and more preferably 1 to 20mg/m².

If solely the carbon film is deposited on the surface of the magneticlayer, the resulting magnetic recording medium may be improvedsignificantly in durability, even although the carbon film is of anextremely thin film thickness. However, it is not possible to preventoutput characteristics after repeated running from being deteriorated.The present invention is characterized by employing not only the carbonfilm, but also the perfluoro polyether derivative as the lubricant. Byhe interaction between the carbon film and the perfluoro polyetherderivative, the recording medium may be improved in running performanceand in durability under a variety of operating conditions.

The above-mentioned perfluoro polyether compound is a new compoundexhibiting superior and long-lasting lubricating properties as comparedto those of conventional lubricant compounds. Besides, the perfluoropolyether derivative exhibits superior lubricating properties when usedunder hostile environments of low temperature and low humidity or,conversely, high temperature and high humidity, such that the compoundis highly useful as the lubricant. Therefore, by employing the perfluoropolyether derivative as a lubricant for the magnetic recording medium,the frictional coefficient of the recording medium may be lowered due toexcellent lubricating effects to improve the running performance,abrasion resistance and durability of the recording mediumsignificantly. Besides, the perfluoro polyether derivative is highlyadvantageous in manufacture because it may be dissolved in solventsother than Freon, such as ethanol.

The perfluoro polyether derivative is a compound of an amine withperfluoro polyether having a terminal carboxylic group(s).

The perfluoro polyethers having a terminal carboxylic group(s) may beemployed without regard to the position of substitution by thecarboxylic groups (COOH). That is, the perfluoro polyethers havingcarboxylic groups (COOH) at both terminal ends or at least one terminalend, may be employed.

The compound of an amine and a perfluoro polyether having carboxylicgroups at both terminals, referred to herein as polyfunctional perfluoropolyether, is represented by the following chemical formula 3. On theother hand, the compound of an amine and a perfluoro polyether having acarboxylic group at one of the terminals referred to herein asmonofunctional perfluoro polyether, is represented by the followingchemical formula 4. ##STR1##

In the above formula, Rf denotes a perfluoro polyether chain and R¹, R²,R³, R⁴ each denote hydrogen or a hydrocarbon residue. ##STR2##

In the above formula, Rf denotes a perfluoro polyether chain and R¹, R²,R³, R⁴ each denote hydrogen or a hydrocarbon residue

As the perfluoro polyethers having terminal carboxylic groups, any ofthe commercially available compounds may be employed.

The monofunctional perfluoro polyethers may be enumerated by

(a) F(CF₂ CF₂ CF₂ O)_(m) CF₂ CF₂ COOH

(b) CF₃ [OCF(CF₃)CF₂ ]_(j) (OCF₂)_(k) COOH

On the other hand, the polyfunctional perfluoro polyethers may beenumerated by

(c) HOOCCF₂ (OCF₂ CF₂)_(p) (OCF₂)_(q) OCF₂ COOH

These compounds are given only for the sake of illustration. In theabove chemical formula for perfluoro polyethers, m, j, k, p and q denoteintegers of not less than unity.

The molecular weight of the perfluoro polyethers having a terminalcarboxylic group(s), for which there is imposed no particularlimitation, is preferably 600 to 5,000 and more preferably 1,000 to4,000. If the molecular fight of the perfluoro polyether becomesexcessive, the effect of the terminal group(s) and the effect thereof asan adsorbent group are diminished, whereas the perfluoro polyetherderivative becomes only difficultly soluble in solvents other thanFreon, such as universal solvents including ethanol. Conversely, if themolecular weight of the perfluoro polyether is too low, the lubricatingeffects due to the perfluoro polyether chain are most.

The amines may be any of primary, secondary or tertiary amines.Quaternary ammonium compounds may also be employed. The structure of theamines may be selected arbitrarily, without regard to the molecularweight or to the presence or non-presence of a branched structure,isomeric structure, alicyclic structure, molecular weight or unsaturatedbonds. The amines preferably contain an alkyl group(s). Above all, theamines may have an alkyl group(s) each having six or more and preferablyten or more carbon atoms for optimum results.

Meanwhile, with the perfluoro polyether derivatives, the perfluoropolyether chains may be partially hydrogenated, that is, a part of (50%or less of) fluorine atoms of the perfluoro polyether chains may bereplaced by hydrogen atoms. In this case, partially hydrogenatedperfluoro polyethers may be employed as the perfluoro polyethers,whereby the amount of consumption of the Freon-based solvent may bediminished. Examples of the partially hydrogenated perfluoro polyethersinclude

(d) F(CF₂ CF₂ CFO)_(a) (CFHCF₂ CF₂)_(b) (CH₂ CF₂ CF₂ O)_(c) CF₂ CF₂ COOH

where a, b and c each denote an integer of not less than unity.

The perfluoro polyether derivative preferably has a molecular weight of1,400 to 4,500. If the molecular weight of the perfluoro polyetherderivative is lower or higher than the above range, the frictionalcoefficient cannot be diminished sufficiently and optimum runningperformance or durability cannot be developed. Besides, outputcharacteristics of the recording medium after repeated running arelowered significantly.

In addition, with the present perfluoro polyether derivative, themolecular weight of a polar group moiety shown by the following chemicalformula 5 is preferably 120 or less. If the molecular weight of thepolar group moiety is larger than 120, the effect thereof as theadsorbent group is lowered due to the weakened interaction between thepolar group moiety and the carbon film due in turn to steric hindranceof a hydrocarbon group contained in the polar group moiety. ##STR3##

In the above formula, R¹, R²,R³, R⁴ each denote hydrogen or ahydrocarbon residue, respectively.

The above perfluoro polyether derivative may easily be synthesized fromthe above perfluoro polyether having a terminal carboxylic group and anamine by the following method.

That is, it may be synthesized by mixing the monofunctional orpolyfunctional perfluoro polyether with an amine and heating theresulting mixture to a temperature exceeding the melting point of theamine used, such as 60° C., when using stearyl amine as an example.

Alternatively, it may be synthesized by dissolving the mono-orpolyfunctional perfluoro polyether in an organic solvent such as Freon,and subsequently eliminating the solvent. If the amine is a quaternaryammonium compound, the perfluoro polyether derivative may be synthesizedby extraction with an organic solvent from a mixture consisting of ametal salt such as a sodium salt of perfluoro polyether, and thequaternary ammonium salt such as chloride, iodide or sulfate.

The amine is coated preferably in an amount of 0.5 to 100 mg/m² and morepreferably in an amount of 1 to 20 mg/m².

When using the perfluoro polyether as a lubricant the perfluoropolyether mixed with amine may be employed as a lubricant without usingthe perfluoro polyether derivative synthesized in advance by theabove-described technique. If the perfluoro polyether mixed with aminebe used, the above-mentioned perfluoro polyether derivative is generatedin situ to display lubricating effects.

The mixing ratio of perfluoro polyether and amine may be set so thatcarboxylic groups constituting a polar group moiety of the perfluoropolyether derivative is equimolar with respect to amine (amino group).However, if the mixture is especially used as a lubricant for themagnetic recording medium, the mixing ratio may be set so that the amineis present slightly in excess for improving the lubricating effects.This is possibly ascribable to the fact that, when the lubricant isapplied to the magnetic layer, the perfluoro polyether exhibitingacidity by the carboxylic group is preferentially adsorbed to themagnetic metal thin film (magnetic layer) which is basic so t hat theamine quantity is in deficit.

Consequently, when the perfluoro polyether derivative is employed as alubricant for the magnetic recording medium, the molar ratio of thecarboxylic group to amine (amino group) (amine/carboxylic group) ispreferably set to 3/7 to 40/1.

Meanwhile, the above-mentioned derivatives may be used singly as alubricant or combined with conventional lubricants.

For more durable lubricating effects under more hostile conditions, anextreme pressure agent may be used in combination at a weight ratio of30:70 to 70:30.

The function of the extreme pressure agent is the friction or abrasioninhibitive action which is achieved by reaction of the extreme pressureagent with a metal surface under the heat of friction on partial metalcontact in a boundary lubricating region resulting in the formation of afilm of a reaction product. Examples of the extreme pressure agentinclude a phosphorus-based extreme pressure agent sulfur-based extremepressure agent halogen-based extreme pressure agent an organometallicextreme pressure agent or a composite extreme pressure agent.

If need be, a rust-proofing agent may be used in compunction with thelubricant and the extreme pressure agent.

The rust-proofing agent may be any of those commonly employed as therust-proofing agent for the magnetic recording medium. Examples of therust-proofing agents include phenols, naphthols, quinones, heterocycliccompounds containing a nitrogen atom(s), heterocyclic compoundscontaining an oxygen atom(s) and heterocyclic compounds containing asulfur atom(s).

The above-described magnetic recording medium according to the firstsubject-matter of the present invention is comprised of a magnetic metalthin film 102 on which are deposited its oxide layer 103 and aprotective layer 104. Our eager searches have revealed that by settingthe film thickness of each of the oxide layer 103 and the protectivelayer 104 so as to be equal to 20 to 230 Å, and by setting the combinedtotal film thickness of the oxide layer 103 and the protective layer 104so as to be equal to 40 to 250 Å, the magnetic recording medium may beimproved significantly in durability without affecting electromagnetictransducing characteristics.

By providing the protective layer 104 on the oxide layer 103, and bysetting the film thicknesses so as to be within the above-definedranges, the film thickness of the oxide layer 3 may be rendered smallerthan with the conventional recording medium while maintaining comparabledurability for thereby improving electro-magnetic transducingcharacteristics.

Amine salts of the perfluoro polyether having a carboxylic group(s) atone or both terminals thereof (perfluoro polyether derivatives) exhibitsuperior lubricating effects to diminish the frictional coefficient. Thelubricating properties of the perfluoro polyether derivative are notlost under stringent conditions of low temperature and/or low humidity.Consequently, by using the perfluoro polyether derivative as a lubricantexcellent durability may be maintained, while running characteristicsmay be improved by the superior lubricating effects.

Besides, by setting the centerline mean roughness R_(a) and the maximumheight of the protrusions R_(max) of the surface opposite to themagnetic layer forming surface of the non-magnetic base film so that0.0015≦R_(a) ≦0.0070 μm and 0.015≦R_(max) ≦0.070 μm, respectively, agood running performance on the outer peripheral surface of the coolingcan of the flexible supporting base film may be realized during vapordeposition of the magnetic layer on the flexible supporting base filmtravelling on the cooling can. Besides, since the flexible base film maybe sufficiently cooled on the cooling can, there is no risk of surfacedegradation due to thermal deterioration or deterioration in productionyield.

On the other hand, by bombarding the magnetic metal thin film, depositedon the base film in a vacuum chamber, at a predetermined making powerdensity, the protective layer may be formed on the magnetic metal thinfilm with a satisfactory bonding power.

By properly selecting the film thicknesses of the oxide layer on themagnetic layer and the protective layer deposited on the oxide layer,the magnetic recording medium may be improved significantly indurability without deteriorating electromagnetic transducingcharacteristics.

Above all, even although the combined total thickness T of the oxidelayer and the protective layer is set to the Order 0f 100 Å of less,with the thicknesses of the respective layers being thus lesser thanwith the conventional recording medium, it becomes possible to improveelectro-magnetic transducing characteristics with maintenance ofsatisfactory durability and to diminish the bit error rate in digitalrecording to improve recording/playback characteristics.

Besides, since the perfluoro polyether derivative (amine salt)exhibiting satisfactory lubricating properties is retained as alubricant on the protective film, the lubricating properties may bemaintained under any operating conditions for an extended period oftime. Consequently, the magnetic recording medium may be provided whichexhibits superior running properties, abrasion resistance and durabilityunder a variety of different operation conditions.

In addition, in the magnetic recording medium according to the presentinvention in which plural magnetic rectal thin films are formed on oneof the surfaces of the flexible base film, the roughness of the runningsurface of the flexible base film is defined by setting the centerlinemean roughness R_(a) and the maximum height of the projections R_(max)so as to be within the ranges of 0.0015≦R_(a) ≦0.0070 μm and0.015≦R_(max) ≦0.070 μm, respectively. Consequently, a good runningperformance on the outer peripheral surface of the cooling can of theflexible supporting base film may be realized during vapor deposition ofthe magnetic layer on the flexible supporting base film travelling onthe cooling can. Besides, since the flexible base film may besufficiently cooled on the cooling can, there is no risk of surfacedegradation due to thermal deterioration, while the energy product errorrate or the electro-magnetic transducing characteristics may beimproved. In addition, there is no risk of creasing due to thermaldeterioration, so that deterioration in production yield is hardlyincurred.

On the other hand, a sufficient bonding power for the protective filmmay be assured by bombardment prior to deposition of the protective filmto inhibit delamination of the protective film during sliding contact ofthe magnetic recording medium with the magnetic head to enable theproduction of a magnetic recording medium superior in durability andoperational reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic enlarged cross-sectional view showing anembodiment of a magnetic recording medium according to the presentinvention.

FIG. 2 is a schematic view showing an arrangement of a vapor depositiondevice.

FIG. 3 is a graph showing a concentration profile along the depth of thecross-section of a typical magnetic recording medium.

FIG. 4 is a graph showing a concentration profile along the depth of thecross-section of another typical magnetic recording medium.

FIG. 5 is a schematic cross-sectional view showing an arrangement of adevice for producing a magnetic recording medium by a method accordingto the present invention.

FIG. 6 is a graph showing a depth profile by Auger electronspectroscopic analysis of a magnetic recording medium produced bybombardment following the deposition of the magnetic metal thin film.

FIG. 7 is a graph showing a depth profile by Auger electronspectroscopic analysis of a magnetic recording medium produced bydeposition of the protective film without bombardment followingdeposition of the magnetic metal thin film.

FIG. 8 is a schematic cross-sectional view showing a typicalconstruction of a magnetic recording medium having, as a magnetic layer,stacked dual-layer magnetic metal thin films with an intermediate oxidelayer in-between.

FIG. 9 is a graph showing a depth profile by Auger spectroscopicanalysis of the magnetic recording medium shown in FIG. 8.

FIG. 10 is a partial schematic side view for illustrating the definitionof the incident angle of a vapor flow in the course of production of themagnetic recording medium shown in FIG. 8.

FIG. 11 is a schematic cross-sectional view showing a typicalconstruction of a magnetic recording medium having, as a recordinglayer, stacked dual-layer magnetic metal thin films after bombardment.

FIG. 12 is a schematic view showing an example of a production deviceemployed for producing the magnetic recording medium shown in FIG. 11.

FIG. 13 is a graph showing the relation between the oxygen gas flowduring vapor deposition and the still time of the magnetic layerproduced by the vapor deposition.

FIG. 14 is a graph showing the relation between the oxygen gas flowduring vapor deposition and the output characteristics and C/Ncharacteristics of the magnetic layer produced by he vapor deposition.

FIG. 15 is a schematic view showing a typical construction of aproducing device for producing a magnetic recording medium having a dualmagnetic layer.

FIG. 16 is a schematic cross-sectional view showing a typicalconstruction of a magnetic recording medium having its reverse surfacecontrolled in surface roughness.

FIG. 17 is a block diagram showing an arrangement of a recording side ofa signal processor of a digital VTR for repression recording of digitalvideo signals in a manner as free from playback distortion as possible.

FIG. 18 is a block diagram showing an arrangement of a playback side ofthe signal processor.

FIG. 19 is a schematic view showing an example of a block for blockencoding.

FIG. 20 is a schematic view for illustrating sub-sampling andsub-lining.

FIG. 21 is a block circuit diagram showing an example of a blockencoding circuit.

FIG. 22 is a block diagram showing an example of a channel encoder.

FIG. 23 is a block diagram showing an example of a channel decoder.

FIG. 24 i s a schematic plan view showing a typical arrangement of arotary drum.

FIG. 25 is a plan view showing a typical construction of the rotary drumand the wrapping state of a magnetic tape.

FIG. 26 is a front view showing a typical construction of the rotarydrum and the wrapping state of a magnetic tape.

BEST MODE OF CARRYING OUT THE INVENTION

Referring to the drawings and results of experiments, illustrativeembodiments of the present invention will be explained in detail.

Scrutiny into Film Thickness of Oxide Layer

A metal magnetic thin film 102, formed of Co₂₀ Ni₈₀ (wt %), wasoblique-deposited in oxygen on a base film 101 formed of polyethyleneterephthalate (PET), as shown in FIG. 1. A typical oblique depositiondevice is explained by referring to FIG. 2.

In FIG. 2, 110 is a vacuum vessel having a centrally disposedcylindrical cooling can 114. A partitioning plate 111 is provided alongthe periphery of the cooling can 114 for dividing the inside of thevacuum vessel 110 into two parts, each of which is evacuated to apredetermined vacuum by evacuating means, not shown, connected toevacuating ports 112, 113. A magnetic recording medium 105, such as amagnetic tape, is adapted for being slidingly guided from a supply roll115 to a take-up roll 116 along the cooling can 114 and guide rolls 117,118 as indicated by arrow a.

An source of evaporation, that is a crucible 121, is provided below theright side of the cooling can 114 in FIG. 2. The material to beevaporated, contained in crucible 121, is bombarded and heated by anelectron beam b from an electron gun 123 so as to be deposited on themagnetic recording medium 105 as indicated by arrow c. 122 is a spareheater for crucible 121.

A shutter 124 is provided below the cooling can 114 for vapor depositionon the magnetic recording medium 105 within a predetermined angularrange. Tn this case, the shutter 124 is provided so that a minimum angleθ_(min) from a direction perpendicular to the film surface of themagnetic recording medium 105 is controlled by the shutter 124 to beequal to e.g. 45°, as indicated by a broken line d.

A gas inlet pipe 125 is positioned so that an oxygen gas, for example,is permeated within the above-mentioned range of deposition. The amountof the oxygen is also controlled so that an oxide 3 is deposited to athickness t₁ on the surface of the magnetic metal thin film 102.

As shown in FIG. 1, a protective layer 104, formed e.g. of carbon, wasdeposited on the oxide layer 103 by e.g. dc magnetic sputtering art anAr gas pressure of 10 retort and a power density of 6.8 W/cm². Theprotective layer 104 was controlled to be of a thickness t₂ bycontrolling the feed rate of the magnetic recording medium 105, herein atape for vapor deposition.

FIGS. 3 and 4 show the profiles of relative concentration along thedepth of the cross-section by Auger electron spectroscope (AES) of themagnetic recording medium 105 not coated with the protective layer 104and the magnetic recording medium 105 of the present embodiment providedwith the protective layer 104. In FIGS. 3 and 4, solid lines e, f, g andb indicate concentrations of carbon, cobalt oxygen and nickel,respectively. It may be sen form FIG. 3 that a non-magnetic cobalt oxideis formed to a larger thickness in the vicinity of the surface of themagnetic recording medium not provided with the protective layer 104.Conversely, with the present embodiments, shown in FIG. 4, theprotective layer 104 of carbon is formed on the major surface, and thecombined total thickness T of the protective layer 104 and the oxidelayer 103 therebelow is on the order of 250≈ which is comprised withinthe range of not less than 40 Å and not more than 250 Å.

A number of samples of the magnetic recording medium 105 were preparedwitch variable thicknesses t₁, t₂ of the oxide layer 103 and theprotective layer 104 and variable total thicknesses T and measurementwas made of the electromagnetic transducing characteristics, error rateand still durability, that is durability during the still mode, of thesesamples. In the following Examples 1 to 6 and Comparative Examples 1 to3, measurements were made with variable thicknesses t₁ and t₂ of therespective layers 3 and 4, with the total thickness T remainingunchanged. The following are the values of the thicknesses t₁ and t₂ andthe total thickness T in the respective Examples.

EXAMPLE 1

The thickness t₁ of the oxide layer 103 was set to 230 Å, while thethickness t₂ of the protective layer 104 was set to 20 Å.

EXAMPLE 2

The thickness t₁ of the oxide layer 103 was set to 200 Å, while thethickness t₂ of the protective layer 104 was set to 50 Å.

EXAMPLE 3

The thickness t₁ of the oxide layer 103 was set to 150 Å, while thethickness t₂ of the protective layer 104 was set to 100 Å.

EXAMPLE 4

The thickness t₁ of the oxide layer 103 was set to 100 Å, while thethickness t₂ of the protective layer 104 was set to 150 Å.

EXAMPLE 5

The thickness t₁ of the oxide layer 103 was set to 50 Å, while thethickness t₂ of the protective layer 104 was set to 200 Å.

EXAMPLE 6

The thickness t₁ of the oxide layer 103 was set to 20 Å, while thethickness t₂ of the protective layer 104 was set to 230 Å.

Comparative Example 1

The thickness t₁ of the oxide layer 103 was set to 250 Å, while thethickness t₂ of the protective layer 104 was set to 0

Comparative Example 2

The thickness t₁ of the oxide layer 103 was set to 240 Å, while thethickness t₂ of the protective layer 104 was set to 10 Å.

Comparative Example 3

The thickness t₁ of the oxide layer 103 was set to 10 Å, while thethickness t₂ of the protective layer 104 was set to 240 Å.

Using a tiled sendust sputter (TSS) type magnetic head, having a gaplength of 0.2 μm and a track width of 20 μm and having its magnetic gapconstituted by a magnetic metal thin film the film surface of which isnon-parallel to the track width of the magnetic gap, theelectro-magnetic transducing characteristics were measured of therespective samples, with the wavelength of 0.5 μm.

Also, using the TSS head having a gap length of 0.2 μm and a track widthof 4 μm, digital receding was performed, and a bit error rate wasmeasured.

The still durability was measured, using an 8-mm VTR manufactured andsold by SONY CORPORATION under the trade name of EV-S1, as the timeuntil the playback output is decreased by 3 dB from an initial output of0 dB, The results of measurement of the above characteristics are shownin Table 1.

                  TABLE 1                                                         ______________________________________                                                         electro-                                                                      magnetic            still                                                     transducing                                                                              bit      dura-                                    thickness [Å]                                                                              characteris-                                                                             error    bility                                   T          t.sub.1                                                                              t.sub.2                                                                              tics [dB]                                                                              rate   (hrs)                                ______________________________________                                        Ex.   1     250    230  20   0        5 × 10.sup.-4                                                                  5                                      2     250    200  50   -0.1     5 × 10.sup.-4                                                                  9                                      3     250    150  100  0        5 × 10.sup.-4                                                                  20                                     4     250    100  150  +0.1     5 × 10.sup.-4                                                                  >20                                    5     250    50   200  0        5 × 10.sup.-4                                                                  >20                                    6     250    20   230  0        5 × 10.sup.-4                                                                  20                               comp. 1     250    250  0    0        5 × 10.sup.-4                                                                  3                                Ex.   2     250    240  10   0        5 × 10.sup.-4                                                                  3.5                                    3     250    10   240  0        5 × 10.sup.-4                                                                  4.5                              ______________________________________                                         *T: total thickness                                                           t.sub.1 : oxide layer                                                         t.sub.2 : protective layer                                               

It is seen from these results that the still durability is as short asthree hours if the protective layer 104 is not provided as inComparative Example 1, whereas, if the protective layer 104 is providedand its thickness t₂ is set to 20 Å or more, the still durability offive hours or longer as required may be achieved. On the other hand, ifthe thickness t₁ of the oxide layer 103 is 10 Å or less, the stilldurability as required cannot be achieved and, if the thickness t₁ is 20Å or more, the still durability on the order of 20 hours may beachieved, as in Example 6. It is also seen that with the total thicknessT remaining constant, the electro-magnetic transducing characteristicsand the error rate remain substantially constant.

In the Examples 7 to 10 and Comparative Examples 4 and 5, respectivesamples of the magnetic recording medium were prepared with variousdifferent total thickness T and thicknesses t₁ and t₂ of the oxide layer103 and the protective layer 104.

EXAMPLE 7

The thickness t₁ of the oxide layer 103, the thickness t₂ of theprotective layer 104 and the total thickness T were set to 50 Å, 150 Åand 200 Å, respectively.

EXAMPLE 8

The thickness t₁ of the oxide layer 103, the thickness t₂ of theprotective layer 104 and the total thickness T were set to 50 Å, 100 Åand 150 Å, respectively.

EXAMPLE 9

The thickness t₁ of the oxide layer 103, the thickness t₂ of theprotective layer 104 and the total thickness T were set to 50 Å, 50 Åand 100 Å, respectively.

EXAMPLE 10

The thickness t₁ of the oxide layer 103, the thickness t₂ of theprotective layer 104 and the total thickness T were set to 20 Å, 20 Åand 40 Å, respectively.

Comparative Example 4

The thickness t₁ of the oxide layer 103, the thickness t₂ of theprotective layer 104 and the total thickness T were set to 50 Å, 250 Åand 300 Å, respectively.

Comparative Example 5

The thickness t₁ of the oxide layer 103, the thickness t₂ of theprotective layer 104 and the total thickness T were set to 20 Å, 10 Åand 30 Å, respectively.

The electro-magnetic transducing characteristics, error rate and thedurability of these samples of the magnetic recording medium weremeasured by the method similar to that shown in Table 1. The results areshown in Table 2.

                  TABLE 2                                                         ______________________________________                                                         electro-                                                                      magnetic            still                                                     transducing                                                                              bit      dura-                                    thickness [Å]                                                                              characteris-                                                                             error    bility                                   T          t.sub.1                                                                              t.sub.2                                                                              tics [dB]                                                                              rate   (hrs)                                ______________________________________                                        Ex.   7     200    50   150  +0.5     4 × 10.sup.-4                                                                  >20                                    8     150    50   100  +1.2     2 × 10.sup.-4                                                                  20                                     9     100    50   50   +1.9     8 × 10.sup.-5                                                                  8                                      10    40     20   20   +2.6     6 × 10.sup.-5                                                                  7                                comp. 1     300    50   250  +0.8     9 × 10.sup.-4                                                                  >20                              Ex.   2     30     20   10   +2.9     5 × 10.sup.-5                                                                  3                                ______________________________________                                         *T: total thickness                                                           t.sub.1 : oxide layer                                                         t.sub.2 : protective layer                                               

It is seen from these results that if the total thickness T is set to avalue exceeding 250 Å, such as 300 Å, the electromagnetic transducingcharacteristics are deteriorated to increase the bit rate, even althoughgood durability is achieved, as shown in Comparative Example 1, whereas,if the total thickness T is diminished gradually to values less than 250Å, the electro-magnetic transducing characteristics are improved tolower the bit error rate significantly. It is also seen that if thetotal thickness T is reduced to a value less than 40 μm, such as 30 μm,the still durability is diminished to three hours, such that desirabledurability is not achieved, even although the electro-magnetictransducing characteristics and the bit error rate are improved.

Consequently, the combined total thickness of the oxide layer 103 andthe protective layer 104 is selected to be not less than 40 μm and notmore than 250 μm. As also seen from Table 1, the respective thicknessesof the layers 103, 104 need to be in excess of 20 Å, so that the upperlimit of each of these layers is selected to be not larger than 230 Å.

Above all, if the total thickness T of the oxide layer 103 and theprotective layer 104 is not larger than approximately 100 Å, that is ifthe thicknesses of the layers 103, 104 are of lower values, thedurability could be maintained at a satisfactory level, while theelectro-magnetic transducing characteristics could be improved and thebit error rate could be improved significantly, as evidenced by Examples9 and 10.

Although the protective layer 104 in the above Examples was formed ofcarbon, the durability could be improved without deteriorating theelectro-magnetic transducing characteristics if, with the use of avariety of inorganic materials, such as SiO₂, Si₃ N₄, SiN_(X), BN orZnO₂, as the material of the protective layer 104, the thickness of theprotective layer is selected appropriately.

The present invention is not limited to the above describedarrangements. For example, the protective layer 104 may be formed oforganic materials, such as ethylene, while the protective layer 104 maybe deposited by a variety of different methods, such as ion plating orplasma CVD.

Scrutiny into Plasma processing of the Surface of Oxide Layer

An arrangement of a producing device employed in the present embodimentis first explained.

Referring to FIG. 5, a partitioning wall X is provided at a mid portionwithin a vacuum chamber 201. The vacuum chamber 201 is divided by thispartitioning wall X into two regions. The region shown to the right inthe drawing is a vacuum deposition chamber for deposition of a magneticmetal thin film, whereas the region shown to the left in the drawing isa sputtering chamber for depositing the protective film.

In an upper part of the vacuum chamber 201, a supply roll 202 and atake-up roll 203 are provided in the vicinity of the opposite lateralsides 201a, 201b, respectively. The supply roll 202 and the take-up roll203 are adapted for being rotated counterclockwise in the drawing, sothat a base film 204 fed out from the supply roll 202 is adapted totravel on its path so as to be taken up by the take-up roll 203.

A cooling can 205, adapted for being rotated clockwise in the drawing,is provided at a mid portion within the vapor deposition chamberprovided with the supply roll 202. The base film 204, fed out from thesupply roll 202, is adapted to travel along the outer periphery of thecooling can 205 at a constant velocity.

Below the cooling can 205 is mounted a crucible 206 within which amagnetic metal material 207 is charged.

Heating means 208 for heating and melting the magnetic metal material207 is provided on the lateral side 201a of the vacuum chamber 201. Thiscauses an electron beam radiated from the heating means 208 to beirradiated on the magnetic metal material 207 to vaporize the magneticmetal material. The vaporized magnetic metal material is then depositedon the base film 204 to form the magnetic metal thin film.

An arcuate shutter 209 is provided along the peripheral surface of thecooling can 205. This shutter covers a part of the surface of the basefilm 204 for limiting the incident angle of the vaporized magnetic metalmaterial 207 on the surface of the base film 207.

A bombarding unit 210 is provided above the partition wall V in thedrawing so that the base film 204 which has traversed the peripheralsurface of the cooling can 205 is supplied to the bombarding unit 210.

The bombarding unit 210 is made up of a pair of bar-shaped electrodes211a, 211b, arranged facing each other, with the base film 204travelling through the inside of the bombarding unit 210 in-between, forbombarding the surface of the magnetic metal thin film formed on thebase film 204 by applying a voltage across these electrodes 211a, 211b.The electrodes 211a, 211b may be of a dc type or an ac type, as desired.

Within the sputtering chamber, provided adjacent to the vacuumdeposition chamber, there is provided a large-diameter can 212 adaptedfor being rotated clockwise in the drawing. The base film 204, which hastraversed the bombarding unit 210, is adapted to travel along the outerperipheral surface of the can 212.

Below the can 212, there is provided a cathode electrode 213, on which atarget 214 is secured. The target 214 is provided facing the peripheryof the can 212 for sputtering on the base film 204 travelling along theouter periphery of the can 212.

Within the sputtering chamber, there is provided a partitioning plate218 at right angles to the partitioning wall X. Sputtering is adapted totake place only within a lower region below the partitioning plate 218.This prevents the sputtering gas from being diffused above thepartitioning plate 218 for improving the sputtering efficiency.

Guide rolls 215a to 215d are provided between the supply roll 202 andthe cooling can 205, between the cooling can 205 and the bombarding unit210, between the bombarding unit 210 and the can 212 and between the can212 and the take-up roll 203, respectively, for applying a predeterminedtension to the base film 204 travelling from supply roll 202 to thetake-up roll 203 for assuring smooth running o the base film 204.

Using the above-described producing device, a magnetic tape was preparedin accordance with the following procedure.

Oblique vapor deposition was carried out on a polyester base film 10 μmin thickness in a vacuum chamber maintained at a predetermined vacuum.

As the base film supplied from the supply side was caused to travelalong the outer surface of the cooling can along the vapor depositionchamber, the magnetic metal material charged in a crucible was heatedand melted by predetermined heating means for vaporizing the magneticmetal material. In the present embodiment the magnetic metal materialwas Co₈₀ Ni₂₀ alloy, with the subscript numbers indicating thecomposition in wt %. The vaporized magnetic material was deposited onthe base film for forming a magnetic metal thin film.

For oblique vapor deposition, an oxygen gas was introduced onto thesurface of the base film at a predetermined rate at a setting of theminimum incident angle of 45° of the magnetic material with respect tothe base film surface. The feed rate of the base film was set so thatthe film thickness of the magnetic metal base film amounted to 200 μm.

The oxide layer on the magnetic metal thin film, thus formed on the basefilm, was bombarded by the bombarding unit. Meanwhile, an Ar gas wasintroduced into the bombarding unit for bombardment.

Then, as the base film which traversed the bombarding unit was run alongthe outer peripheral surface of the can provided below the bombardingunit a carbon film was deposited as a protective film to a filmthickness of 100 Å on the magnetic metal thin film by dc magnetronsputtering under a continuous take-up system.

For sputtering, the vacuum in the sputtering chamber was set to 2 Pa. Atarget (carbon) secured to the cathode electrode was arranged at adistance of 8 cm from the peripheral surface of the can. Sputtering wascarried out as the base film was run at a feed rate of 2 m/sec. Themaking power of the cathode electrode was set to 6.8 W/cm².

For checking into the effect of the bombardment operation, Augerelectron spectroscopic analysis along the depth from the magnetic tapesurface was carried out using a magnetic tape which could be obtainedwith the making power density during bombardment of 18.9 kW/m².

It was indicated by this analysis that since an oxygen-containing regionC was present between a region A in the vicinity of the magnetic tapesurface formed mainly of carbon (region corresponding to the protectivefilm) and a region B formed mainly of Co and oxygen (regioncorresponding to the magnetic metal thin film), an oxidized surfacelayer should be interposed between the protective film and the magneticmetal thin film, as shown in FIG. 6. The oxidized surface film had afilm thickness o approximately 130 Å, with the combined thicknessthereof with he protective film being 230 Å.

By way of comparison, a protective film was formed after deposition ofthe magnetic metal thin film without carrying out a bombardmentoperation, and an Auger electron spectroscopic analysis was carried outalong the depth from the tape surface. The analysis revealed that theoxidized surface layer was interposed between the protective film andthe magnetic metal thin film, as when the bombardment operation wascarried out as shown in FIG. 7. However, in this case, the filmthickness of the oxidized surface layer amounted to approximately 200 Å,with the total film thickness with the protective film being 300 Å.

It was now seen from the above results that the bombardment operationled to the reduction in thickness of the oxidized surface layer.

Scrutiny was then made into the durability and the electromagnetictransducing characteristics of the above described magnetic tape.

That is, the still durability and the playback output for the wavelengthof 0.5 μm were checked of magnetic tapes produced with the making powerdensities during bombardment of 1.6 kW/m², 11.8 kW/m² and 18.9 kW/m¹(Examples 1 to 31), a magnetic tape in which, for comparison sake, aprotective film was formed without bombardment after deposition of themagnetic metal thin film (Comparative Example 6), a magnetic tape inwhich the bombardment operation was carried out in an oxygen gasatmosphere (Comparative Example 7) and a magnetic tape in which themaking power density during bombardment was set to 1.11 kW/m²(Comparative Example 8). The results are shown in Table 3.

Meanwhile, the still durability was evaluated based on the time elapseduntil the playback output deteriorated by 3 dB from an initial value inthe course of still running on the remodelled machine of EV-Si by SONYCORPORATION at a relative velocity of 7.5 m/sec between the magnetichead and the medium.

                  TABLE 3                                                         ______________________________________                                        bombarding conditions                                                                           making                play-                                                   power          still  back                                  voltage   current density        durability                                                                           output                                (V)       (Å) (kW/m.sup.2)                                                                           gases (hrs)  (dB)                                  ______________________________________                                        Ex. 11                                                                              200     0.1      1.6   Ar    12     +0.2                                Ex. 12                                                                              300     0.5     11.8   Ar    >20    +0.5                                Ex. 13                                                                              400     0.6     18.9   Ar    >20    +0.7                                comp. --      --      --     none  0.5    0                                   Ex. 6                                                                         comp. 300     0.5     11.8   O.sup.2                                                                             >20    -0.8                                Ex. 7                                                                         comp. 180      0.08    1.1   Ar    1.5    0                                   Ex. 8                                                                         ______________________________________                                    

It is seen from Table 3 that if the surface of the magnetic metal thinfilm, produced by vapor deposition as in the present embodiment thebonding power of the protective film to the magnetic metal thin film isimproved to assure satisfactory still durability. Also the oxidizedsurface layer formed on the magnetic metal thin film is reduced inthickness to diminish the spacing losses to improve the playback outputsignificantly.

It has also been found that if the above-mentioned bombardment operationis not carried out of if the making power density during bombardment islow, the bonding power of the protective film cannot be increased, whilethe still durability cannot be improved. On the other hand, if thebombardment is carried out in an oxygen gas atmosphere, the surface ofthe magnetic metal thin film keeps on to be oxidized so thatconsiderable deterioration is incurred in the output characteristics dueto spacing loses, even although the durability was improved to someextent.

Meanwhile, if the bombardment operation is carried out directly afterthe deposition of the magnetic metal thin film by the oblique vapordeposition in the vacuum chamber, the produced magnetic tape is oncetaken up and a protective film is formed thereon in a separate vacuumchamber to depart from an in-line operation (Comparative Example 4), asufficient bonding power between the protective film and the magneticmetal thin film cannot be developed so that the still durability wassignificantly lowered.

Scrutiny into Intermediate Oxidized Layer in Multilayered Structure

EXAMPLE 14

In the present Example, two magnetic metal thin films, each having aninclined columnar structure grown in a forward direction relative to aline normal to the non-magnetic base film, are formed by oblique vapordeposition on the non-magnetic base film, with an intermediate layerinterposed between these thin films, for producing a forward two-layeredtype magnetic tape.

The magnetic tape in the present embodiment is comprised of anon-magnetic base film 301 of polyethylene terephthalate and a firstmagnetic metal thin film 302 formed thereon to a thickness of 900 Å, asshown in FIG. 8. The first magnetic metal thin film 302 has an inclinedcolumnar structure in which the column is inclined the more pronouncedlyrelative to the line normal to the base film 301 the closer the columnis to the base film 301 and the column is inclined the less pronouncedlyrelative to the normal line the further away the column is from the basefilm 301.

An intermediate layer 3 formed of a cobalt oxide film is provided on thefirst magnetic metal thin film 302. The film thickness of the firstintermediate layer 303 is approximately 200 Å.

A second magnetic metal thin film 304 is formed on the firstintermediate layer 303 to a thickness of 900 Å. The direction of growthof the inclined columnar structure of the second magnetic metal thinfilm 304 is the same as that of the first magnetic metal thin layer 302.

With the above-described magnetic tape, the first magnetic metal thinfilm 302 and the second magnetic metal thin film 304 are stacked withthe first intermediate layer 303 in-between, by way of the recordinglayer, and has a two-layered structure, as long as the magnetic metalthin film structure is concerned. The total thickness of the recordinglayer is 2000 Å, with the thickness of the first intermediate layer 303accounting for 10% of the total film thickness.

The method for producing the magnetic tape is hereinafter explained.

A non-magnetic base film 312 was wrapped around the outer peripheralsurface of a drum 311 and, as the base film 312 was moved in a directionshown at a in FIG. 9 with rotation of the drum 311, a vapor stream froman evaporation source 314 charged in a crucible 313 was impinged on thebase film 312 at an angle of incidence relative to a line normal to thebase film 312 for depositing the magnetic metal material on thenon-magnetic base film 312, as shown in FIG. 9. A pair of shutters 306,306 were provided which are opened in a range of from the maximum angleof incidence θ₁ to a minimum angle of incidence θ₂. The obliquedeposition was started at the maximum angle of incidence θ₁ and theangle of incidence was continuously changed with movement of thenon-magnetic base film 312 to carry out vapor deposition on an area ofthe base film 312 exposed to the shutters 316, 316 up to a regiondelimited by the minimum angle of incidence θ₂ for forming a magneticmetal thin film 315 on the non-magnetic base film 312.

Then, as the non-magnetic base film 301 was moved in one direction, avapor stream from the evaporation source (pure Co) was impinged on thenon-magnetic base film 301 at a predetermined angle of incidence and, asthe angle of incidence was changed continuously, the oblique vapordeposition was carried out to deposit the first magnetic metal thin film302 on the non-magnetic base film 301.

Then, using pure Co as a target the first intermediate layer 303 wasformed by magnet ton sputtering on the first magnetic metal thin film302. The sputtering conditions were an O₂ gas low rate of 200 SCCM, anO₂ flow rate of 70 SCCM and the running velocity of the non-magneticbase film of 10 m/min.

The second magnetic metal thin film 304 was formed on the firstintermediate layer 303 by the oblique vapor deposition similar to thatperformed for the first magnetic metal thin film 302, for producing amagnetic tape.

For analyzing the composition of the recording layer, the depth profilewas analyzed, using an Auger electron spectroscopic device manufacturedand sold by NIPPON DENSHI KK under the trade name of Jamp 30. It wasrevealed from the analysis that since a peak of oxygen atoms appears inthe vicinity of the depth of 1000 Å from the tape surface, with relativedecrease in Co peak, as shown in FIG. 10, the first intermediate layer303 of a cobalt oxide film was formed between the first magnetic metalthin film 302 and the second magnetic metal thin film 304. The filmthickness of the first intermediate layer 303 was certainly approximateto 200 Å, as ascertained by the half value width l of the peak of theoxygen atom in the vicinity of 1000 Å (the width of peak at height h/2).Meanwhile, an oxide layer having a film thickness of 50 Å was formed onthe surface of the second magnetic metal thin film.

A back-coat layer was formed by a usual technique on the oppositesurface of the non-magnetic base film. The magnetic tape was completedby formation of a top coat layer comprised of a carbon protective layer(150 Å) and a lubricant followed by a rust-proofing process.

EXAMPLE 15

The forward two-layered type magnetic tape was prepared in the same wayas in Example 14 except using the film thickness of each of the firstand second magnetic metal thin films of 950 Å and the film thickness ofthe intermediate layer of 100 Å. The ratio of the thickness of theintermediate layer to the total thickness (2000 Å) was set to 5%.

EXAMPLE 16

A forward three-layered magnetic tape was prepared in the same manner asin Example 1, except forming a third magnetic metal thin film on thesecond magnetic metal thin film with the interposition of a secondintermediate layer 200 Å thickness with the direction of growth of theinclined columnar structure of the third magnetic metal thin film beingthe forward direction with respect to the direction of growth for thefirst and second magnetic metal thin films.

Meanwhile, the second and the third magnetic metal thin films weredeposited by the same method as that used for the deposition of thefirst intermediate layer and the first magnetic metal thin film inExample 14.

The film thickness of each of the magnetic metal thin films was set to530 Å, with the ratio of the total thickness of the two layers to thetotal thickness (2000 Å) of the recording layer being 20%.

EXAMPLE 17

A reverse two-layered magnetic tape was prepared in the same way as inExample 1, except setting the direction of growth of the inclinedcolumnar structure of the second magnetic metal thin film so as to bereversed from that of the first magnetic metal thin film.

That is, the second magnetic metal thin layer was formed by running thenon-magnetic base film in the opposite direction to that in which thetape was run during formation of the first magnetic metal thin film.

The thickness of the first intermediate layer was set to 200 θ, whichaccounted for 10% of the total thickness of the recording layer (2000Å).

Comparative Example 9

A single-layer type magnetic tape was prepared by depositing a firstmagnetic metal thin film, as a sole recording layer, 2000 θ inthickness, on a non-magnetic metal thin film, by the same technique asthat used in Example 14.

Comparative Example 10

For comparison sake, a forward two-layered magnetic tape was prepared inthe same way as in Example 14 except using the thickness of theintermediate layer of 500 Å and the ratio of the thickness of theintermediate layer to the total thickness of the recording layer (2000Å) of 25%.

Output characteristics and the C/N ratio of the magnetic produced asabove were checked. The results are shown in Table 4.

Meanwhile, by way of measuring the output characteristics, the playbackoutputs of the respective magnetic tapes were measured using an 8-mmvideo tape recorder manufactured and sold by SONY CORPORATION under thetrade name of EVS-900, with the wavelength of input signals being 0.5μm. The C/N ratio was measured using the above 8-mm video tape recorderEVS-900 by SONY CORPORATION, with the wavelength of input signals being0.5 μm. The measured values were indicated as relative values for thevalue of magnetic tape of Comparative Example 9 of 0 dB.

                  TABLE 4                                                         ______________________________________                                                    thickness (Å)                                                                        rate (%) of                                                                              output                                      recording   of each    total thick-                                                                             charac-                                                                             C/N                                   layer       intermediate                                                                             ness of inter-                                                                           teristic                                                                            ratio                                 structure   layer      mediate layer                                                                            (dB)  (dB)                                  ______________________________________                                        Ex. 14                                                                              forward   200        10       +1.6  +2.5                                      two layers                                                              Ex. 15                                                                              forward   100         5       +2.0  +2.6                                      two layers                                                              Ex. 16                                                                              forward   200        20       +2.0  +2.9                                      three layers                                                            Ex. 17                                                                              reverse   200        10       +0.9  +1.2                                      two layers                                                              comp. sole layer                                                                              --         --       0     0                                   Ex. 9                                                                         comp. forward   500        25       -1.0  +1.0                                Ex. 10                                                                              two layers                                                              ______________________________________                                    

It was seen that, with the magnetic tape in which the ratio of thecombined total thickness of the intermediate layers to the totalthickness of the recording layer is set so as to be 20% or less,satisfactory results could be obtained with respect to outputcharacteristics and the C/N ratio, as shown in Table 4,

EXAMPLE 18

In the present Example, a forward two-layered magnetic tape wasprepared, in which, by changing the angle of incidence of a vapor streamto a line normal to a non-magnetic base film in a range of from 40° to70°, two magnetic metal thin films were deposited on the base film sothat the inclined columnar structures thereof were grown in the samedirection.

First an undercoat film having minute protrusions was formed on one ofthe surfaces of a non-magnetic base film of polyethylene terephthalateof 10 μm thickness by coating an undercoating solution, such as anacrylic emulsion thereon. Then, as the base film was run in onedirection, a vapor stream from an evaporation source was impingedthereon so that the angle of incidence relative to a line normal to thebase film decreased continuously, by way of an oblique vapor deposition.A first magnetic metal thin film was deposited on the non-magnetic basefilm to a thickness of 1000 Å, until the minimum angle of incidence of40° was reached, with the maximum angle of incidence at the start of thevapor deposition being 70°. For the oblique vapor deposition, a Co₈₀Ni₂₀ alloy was used as source of evaporation, and the O₂ gas flowintroduced into the atmosphere of evaporation was set to 200 SCCM, withthe running speed of the non-magnetic base film being set to 10 m/sec.The oblique vapor deposition was carried out repeatedly under the sameconditions for depositing a second magnetic metal thin film on the firstmagnetic metal thin film. If the running direction of the base film isthe same as that used for vapor deposition of the first magnetic metalthin film, the direction of growth of the inclined columnar structure ofthe second magnetic metal thin film is the same as that of the firstmagnetic metal thin film. Meanwhile, an oxide layer was formed on thesurface of the second magnetic metal thin film to a thickness of 50 Å.

A back-coat layer was hen formed on the opposite surface of thenon-magnetic base film. After forming a top coat layer consisting of aprotective carbon layer (150 Å) and a lubricant, a rust-proofingoperation was carried out under prescribed conditions for completing amagnetic tape.

EXAMPLE 19

A magnetic tape was prepared in the same way as in Example 18 exceptchanging the maximum angle of incidence and the minimum angle ofincidence to 65° and 45°, respectively.

Comparative Example 11

For comparison, oblique vapor deposition was carried out for depositinga magnetic metal thin film with the range of change of the angle ofincidence being increased to 90°.

That is, a magnetic tape was prepared in the same way as in Example 18,except changing the maximum angle of incidence to 90°, with the minimumangle of incidence θ₂ of 40° remaining the same.

Output characteristics and frequency characteristics were checked of theExamples 18, 19 and the Comparative Example 11. The results are shown inTable 5.

The output characteristics are measured values of the playback outputfor the input signal wavelength of 0.5 μm, indicated as relative valueswith respect to the corresponding values for Comparative Example 11. Thefrequency characteristics are relative values with respect toComparative Example 3 for indicating to which extent the attenuation ofthe playback output at 10 MHz has been compensated relative to theplayback output at 5 MHz.

                  TABLE 5                                                         ______________________________________                                                output characteristics                                                                      frequency                                                       (dB)          characteristics (dB)                                    ______________________________________                                        Ex. 18    +0.5            +2.1                                                Ex. 19    +0.7            +2.5                                                Comp. Ex. 11                                                                            0               0                                                   ______________________________________                                    

It is seen from Table 5 that satisfactory output characteristics may beobtained in accordance with the present invention. It is also seen thatif Comparative Example 11 is taken as a reference, the attenuation ofthe playback output at 10 MHz is suppressed as compared to that at 5MHz.

EXAMPLE 20

In the present Example, the oblique vapor deposition was carried out forforming forward two-layered magnetic metal thin films on thenon-magnetic base film, with the interposition of an oxide intermediatelayer between the magnetic metal thin films, by continuously changingthe angle of incidence of the vapor stream with respect to the linenormal to the non-magnetic base film from 70° to 40°.

As the non-magnetic base film of polyethylene terephthalate of 10 μmthickness, on which a coating film of an undercoat solution, such as anemulsion, was applied in advance, was run at a rate of 10 m/sec in onedirection, oblique vapor deposition was carried out in the same way asin Example 18 for forming a first magnetic metal thin film of 900 Åthickness on the non-magnetic base film. For the oblique vapordeposition, the maximum angle of incidence θ₁ and the minimum angle ofincidence were θ₂ set to 70° and 40°, respectively. Magnetron sputteringwas then carried out in the same way as in Example 14 for depositing acobalt oxide intermediate layer on the first magnetic metal thin film toa thickness of 200 Å.

Oblique vapor deposition was then carried out in the same way as for thefirst magnetic metal thin film for depositing a second magnetic metalthin film on the intermediate layer to a film thickness of 900 Å.

The recording layer of the magnetic tape, prepared in this manner, is ofa dual-layered structure consisting of the first and second magneticmetal thin films, deposited on the non-magnetic base film with theintermediate film in-between, with the total thickness being 2000 Å. Thethickness of the first intermediate layer accounts for 10% of the totalthickness of the recording layer. Besides, an oxide layer was formed onits surface to a thickness of 50 Å.

A back-coat layer was then formed on the opposite surface of thenon-magnetic base film. After forming a top coat layer consisting of aprotective carbon layer (150 Å) and a lubricant a rust-proofingoperation was carried out under prescribed conditions for completing amagnetic tape.

EXAMPLE 21

A magnetic tape was prepared in the same way as in Example 20, exceptchanging the maximum angle of incidence to 65° and setting the runningvelocity of the non-magnetic base film to 20 m/sec.

Meanwhile, the thickness of the intermediate layer was set to 100 Å,while the ratio of the thickness of the intermediate layer to the totalthickness of the recording layer was set to 5%.

Using the magnetic tapes produced in Examples 20 and 21, outputcharacteristics and the C/N ratio were checked in accordance with heabove method. The results are shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                           Ex. 20 Ex. 21                                              ______________________________________                                        vapor stream incidence angle (°)                                                            θ.sub.1 = 40                                                                     θ.sub.1 = 40                                                   θ.sub.2 = 70                                                                     θ.sub.2 = 65                              intermediate layer thickness (Å)                                                               200      100                                             tape running speed (m/min)                                                                         10       20                                              output characteristics (dB)                                                                        +2.1     +2.6                                            C/N rate (dB)        +2.8     +2.9                                            ______________________________________                                    

It is seen from Table 6 that on comparison of the above results of Table6 in conjunction with the results of Example 14 and 15, the outputcharacteristics and the C/N ratio obtained when the angle of incidenceof the vapor stream to the line normal to the non-magnetic base film isregulated to a range as defined by the present invention as in Example20 or Comparative Example 21 are superior to those obtained with simpleoblique vapor deposition.

Effects of Bombardment in Multilayered Structure

With the magnetic tape of the present embodiment as shown in FIG. 11, adual-layered magnetic layer consisting of a first magnetic thin film 40and a second magnetic thin layer 403 is formed on one of the majorsurfaces of a non-magnetic base film 401 formed of polyethyleneterephthalate.

On the major surface of the non-magnetic base film 401, a coating filmof acrylate-based high molecular latex is formed as an undercoat for thefirst magnetic thin film 402, and the first magnetic thin film 402 isformed on the non-magnetic base film 401 via the undercoat film. Fineparticles with a mean particle size of 400 Å are contained in theundercoat layer and are present at a rate of 10,000,000 particles permm².

The first magnetic thin film 402 is formed on the undercoat film, andthe second magnetic thin film 403 is formed on the first magnetic thinfilm 402. These magnetic thin films 402, 403 are deposited by theoblique vapor deposition method with the growth direction being the sameforward direction. The magnetic thin films 402, 403 are each 1,000 Å inthickness.

The surface of the first magnetic thin film 402 is partially oxidized sothat the film 401 is magnetically isolated from the second magnetic thinfilm 403 deposited thereon. The oxidized state of the surface of thefirst magnetic thin film 402 is realized by introducing an oxygen gasinto an atmosphere of oblique vapor deposition.

The surface of the first magnetic thin film 402 is bombarded duringvapor deposition as will be explained subsequently. This reduces thethickness of or eliminates the oxidized layer formed on the firstmagnetic thin film during the oblique vapor deposition to preventdeterioration of the electro-magnetic transducing characteristics of themagnetic layer otherwise caused by the oxidized layer.

On the other hand, an oxidized layer (50 Å) and a top coat layer 404consisting of a carbon protective film (150 Å) and perfluoro polyetherare formed on the second magnetic thin film 403.

A back coat layer 405 formed of carbon and urethane is formed on theopposite major surface of the non-magnetic base film 401.

The above-described magnetic tape may be produced by the followingproducing device.

With the present producing device, a feed roll 413 adapted to be rotatedcounterclockwise at a constant velocity and a take-up roll 414 adaptedto be rotated clockwise at a constant velocity are provided within avacuum chamber 411 which is maintained at vacuum by being evacuated viaupper and lower evacuation ports 423, as shown in FIG. 12. A tape-shapednon-magnetic base film 412 is adapted to travel from the feed roll 413up to the take-up roll 413.

A cooling can 415 larger in diameter than the rolls 413, 414 is providedhalfway on a travel path of the non-magnetic base film 412. The coolingcan 415 is provided for pulling out the base film 412 downwards in thedrawing, and is adapted for being rotated clockwise at a constantvelocity. Meanwhile, the feed roll 413, take-up roll 414 and the coolingcan 415 are in the form of cylinders each having the same height as thatof the base film 412. A cooling unit not shown, is provided within thecooling can 415 for inhibiting deformation of the non-magnetic base film412 due to rise in temperature.

Thus the non-magnetic base film 412 is adapted for being supplied fromthe feed roll 413 so as to travel around the periphery of the coolingcan 415 until it is taken up by the take-up roll 414. Guide rolls 416,417 are provided between the can feed roll 413 and the cooling can 415and between the cooling can 415 and the take-up roll 414, respectively,for applying a predetermined tension to the non-magnetic base film 412travelling between the feed roll 413 and the cooling can 415 and betweenthe cooling can 415 and by the take-up roll 414, respectively, forallowing smooth running of the base film 412.

A crucible 418 is provided below the cooling can 415 within the vacuumchamber 411 and a magnetic metal material (Co₈₀ Ni₂₀) 419 is charged incrucible 418. The crucible 418 is substantially equal in width to thecooling can 415.

On a lateral wall of the vacuum chamber 411 is mounted an electron gun420 for heating and evaporating the magnetic metal material 419 chargedin crucible 418. The electron gun 420 is provided at such a positionthat an electron beam X radiated from the electron gun 420 is radiatedon the magnetic metal material 419 within crucible 418. The magneticmetal material 419 evaporated by the electron gun 420 is deposited as amagnetic layer on the non-magnetic base film 412 adapted to travel at aconstant velocity around the peripheral surface of the cooling can 415.

A shutter 422 is provided in the vicinity of the cooling can 415 forextending along the peripheral surface of the cooling can. This resultsin the angle of incidence of the magnetic metal material 419 on thesurface of the non-magnetic base film 412 being regulated because thesurface of the base film 412 is partially covered by the shutter 422.

Consequently, as the magnetic metal material 419 charged in crucible 418is heated and evaporated by the electron gun 419 so as to be depositedon the non-magnetic base film 415 travelling along the peripheralsurface of the cooling can 415, deposition on the non-magnetic base film412 of the evaporated magnetic metal material 419 is terminated at aregion which is reached a time point when the non-magnetic base film 412starts to be covered by one end of the shutter 422, that is a feed-outend of the on-magnetic base 412, with the angle of incidence θ₁ of themagnetic metal material 419 evaporated at this time being a minimumangle of incidence with respect to the non-magnetic base film 412.

A partitioning plate 421 is provided within the vacuum chamber 411 fordividing the cooling can 415 into an upper portion and a lower portion.In this manner, deposition on the non-magnetic material 412 fed out fromthe feed roll 413 is started at a time point when the base film ispassed by the partition plate 421, with the angle of incidence θ₂ of themagnetic metal material 419 evaporated at this time point becoming amaximum angle of incidence with respect to the non-magnetic base film412. Also, by providing such partitioning plate 421, dispersion of theevaporated magnetic metal material 419 to a space above the partitionplate 421 is inhibited to improve the evaporation efficiency.

An oxygen gas inlet 428 is provided through the lateral wall of thevacuum chamber 411 for providing an oxygen gas during evaporation on thesurface of the non-magnetic base film 412 via the inlet 428 onto thesurface of the non-magnetic base film 412. This allows oxygen to becaptured by the resulting magnetic thin film to improve magneticcharacteristics.

Also, the present producing device is provided with processing units424, 425 halfway between the guide rolls 416, 417 on the feed-out andtake-up sides of the non-magnetic base film 412 and the cooling can 415,respectively.

These processing units 424, 425 are provided for bombarding the surfaceof the non-magnetic base film 412 and the surface of a magnetic thinfilm formed by oblique vapor deposition as described above,respectively. Before and after the deposition of the magnetic thin film,the magnetic tape having the non-magnetic base film 412 and the magneticthin film formed thereon is adapted to pass through the inside of theprocessing units 424, 425, respectively, for carrying out bombardment onthe surface of the non-magnetic base film 412 and that of the magneticthin film.

Thus the non-magnetic base film 412 fed out from feed roll 413 is passedthrough the inside of the processing unit 424 to travel on theperipheral surface of the cooling can 415 and through the inside of theprocessing unit 425 so as to be taken up on the take-up roll 414.

The processing units 424, 425 are provided with inlets 424a, 425a andoutlets 424b, 425b, for allowing passage of the non-magnetic base film412 and the magnetic tape having the magnetic thin film formed thereon,respectively, so that the surface of the non-magnetic base film 412 andthat of the magnetic tape having the magnetic thin film formed thereonare bombarded since a time point of entrance via the inlets 424a, 425ainto the processing units 424, 425 until a time point of exit via theoutlets 424b, 425b. Such bombardment operation results in reduction inthickness of elimination of the oxide layer formed on the non-magneticbase film 412 or the magnetic thin film produced by vapor depositionthereon to prevent deterioration in electro-magnetic transducingcharacteristics otherwise caused by the presence of the oxide layers.

Within the processing unit 424 (or 425), there are provided a pair ofbar-shaped electrodes 426, 426 (or electrodes 427, 427) via non-magneticbase film 412 (or the magnetic tape), so that an electrical discharge isincurred between these electrodes 426, 426 (or electrodes 427, 427).These electrodes 426, 426 (or electrodes 427, 427) may be of the dc orac type, as desired.

An inert gas containing or not containing a reducing gas is introducedinto these processing units 424, 425 for water-cooling the electrodes426, 426 (or electrodes 427, 427).

Meanwhile, with these processing units, bombardment is carried out notonly on the surface of the magnetic thin film formed by the obliquedeposition, but also on the surface of the non-magnetic base film 412.However, it suffices if at least the surface of the magnetic thin filmis bombarded, while the bombardment of the surface of the non-magneticbase film 412 may be omitted.

Using the above-described producing device, a variety of magnetic tapeswere produced in accordance with the following procedure.

EXAMPLE 22

An acrylate-based high molecular latex, having a mean particle size of400 Å, was applied on one major surface of a polyethylene terephthalatefilm, having a thickness of 10 μm, to form an undercoat layer, at a rateof 1,000,000 particles per mm².

Using a Co₈₀ Ni₂₀ alloy, wherein the subscripts denote the proportionsof composition, as a magnetic metal material, a polyethyleneterephthalate film, having the undercoat layer formed thereon, was runat a tape velocity of 30 m/sec, at the same time that an oxygen gas wasintroduced, by way of carrying out oblique vapor deposition on thepolyethylene terephathalate film, for forming a first magnetic thin filmon the polyethylene terephthalate film to a film thickness of 1,000 Å.

The amount of the oxygen gas introduced was set to 200 cc/min, while theangle of incidence of the evaporated magnetic metal material on thepolyethylene terephthalate film surface was changed within the range of45° and 90°.

In the course of the vapor deposition, the surface of the polyethyleneterephthalate film and that of the produced first magnetic thin filmwere bombarded. The conditions of the bombardment included the voltageand the current of the electrodes in an Ar gas atmosphere on thepolyethylene terephthalate film surface of 500 V and 0.2 Å,respectively, and the voltage and the current of the of the electrodesin an atmosphere containing 5% of an H₂ gas on the first magnetic thinfilm surface of 500 V and 0.3 Å, respectively.

After the polyethylene terephthalate film, taken up on the take-up roll,was again reeled out a second magnetic metal thin film was deposited toa film thickness of 1,000 Å, similarly to the first magnetic thin film,while the surface of the produced second magnetic thin film wasbombarded. The same bombardment conditions as those for the polyethyleneterephthalate film surface were employed.

After the vapor deposition, a back-coat layer composed of carbon andurethane and a top coat layer composed of perfluoro polyether wereformed on the produced magnetic tape. Besides, the magnetic tape was cutto a width of 8 mm.

EXAMPLE 23

A magnetic tape was prepared in the same way as in Example 22, exceptthat the Ar gas atmosphere containing 5% of an H₂ gas employed for thebombardment operations for the first magnetic thin film in Example 22was replaced by an Ar gas atmosphere containing 4% of acetylene.

EXAMPLE 24

A magnetic tape was prepared in the same way as in Example 22 exceptthat the voltage of 500 Å and the current of 0.3 Å employed for thebombardment of the first magnetic thin film were changed to the voltageof 500 V and the current of 0.05 Å, respectively.

EXAMPLE 25

A magnetic tape was prepared in the same way as in Example 22 exceptthat the voltage of 500 Å and the current of 0.3 Å employed for thebombardment of the first magnetic thin film were changed to the voltageof 500 V and the current of 0.9 Å, respectively.

Comparative Example 12

A magnetic tape was prepared in the same way as in Example 22, exceptthat the AF gas atmosphere containing 5% of an H₂ gas employed for thebombardment operations for the first magnetic thin film in Example 22was replaced by an AF gas atmosphere not containing a reducing gas.

For each of these magnetic tapes, a playback output the C/N ratio andthe error rate at a wavelength of 0.54 μm were measured, using a deviceEVS-900 manufactured by SONY CORPORATION. The results are shown in Table7. It is noted that values of the constant K indicating the bombardmentprocessing capability for the first magnetic thin films of therespective magnetic tapes (see formula (1) above) are also entered inTable 7.

                  TABLE 7                                                         ______________________________________                                               constant K                                                                             playback                                                             for      output                                                               bombardment                                                                            (dB)     C/N (dB)  error rate                                 ______________________________________                                        Ex. 22   39.4       +1.2     +2.1    7.6 × 10.sup.-5                    Ex. 23   39.4       +1.4     +1.9    8.1 × 10.sup.-5                    Ex. 24   6.56       +0.3     +0.5    7.8 × 10.sup.-4                    Ex. 25   118.1      +1.4     +2.0    8.0 × 10.sup.-5                    Comp. Ex. 12                                                                           26         0        0       2.9 × 10.sup.-4                    ______________________________________                                    

It is seen from Table 7 that in Examples 22 to 25, the playback outputand the C/N ratio are higher and the error rate is lower than inComparative Example. Consequently, superior durability and improvedelectro-magnetic transducing characteristics may be achieved by carryingout the bombardment operation during vapor deposition, as describedabove.

However, if the value of the constant K during the bombardment is smallas in Example 24, it has not been possible to improve the playbackoutput the C/N ratio of the error rate sufficiently. It may be said fromthis that the value of the constant K for the bombardment operation onthe order of 10 or more is desirable.

Lubricating effects by Amine Salts of Perfluoro Polyester

The durability and the running performance under various conditions ofuse were checked of evaporated tapes in which a lubricating layer wasformed on a carbon film formed on the surface of a magnetic layer usinga perfluoro polyether derivative shown by the following Formulas 6 or 7:##STR4## (where Rf denotes a perfluoro polyether chain and R¹, R², R²and R⁴ each denote hydrogen or a hydrocarbon residue) ##STR5## (where Rfdenotes a perfluoro polyether chain and R¹, R², R³ and R⁴ each denotehydrogen or a hydrocarbon residue).

Experiment 1

The following magnetic recording media were prepared using 14 differenttypes of perfluoro polyether derivatives (Compounds 1 to 14). It isnoted that the main chains constituting the perfluoro polyetherderivatives (compounds 1 to 14) employed in the present experiment thatis the perfluoro polyether chains having terminal carboxylic group(s),the hydrocarbon groups or hydrogen contained in the polar group moiety,and the molecular weights, are as shown in Tables 8 and 9.

                  TABLE 8                                                         ______________________________________                                        compound name   perfluoro polyether chains                                    ______________________________________                                        compound 1      CF.sub.2 (OC.sub.2 F.sub.4).sub.P (OCF.sub.2).sub.q                           OCF.sub.2                                                     compound 2      CF.sub.2 (OC.sub.2 F.sub.4).sub.P (OCF.sub.2).sub.q                           OCF.sub.2                                                     compound 3      CF.sub.2 (OC.sub.2 F.sub.4).sub.P (OCF.sub.2).sub.q                           OCF.sub.2                                                     compound 4      CF.sub.2 (OC.sub.2 F.sub.4).sub.P (OCF.sub.2).sub.q                           OCF.sub.2                                                     compound 5      CF.sub.2 (OC.sub.2 F.sub.4).sub.P (OCF.sub.2).sub.q                           OCF.sub.2                                                     compound 6      CF.sub.2 (OC.sub.2 F.sub.4).sub.P (OCF.sub.2).sub.q                           OCF.sub.2                                                     compound 7      CF.sub.2 (OC.sub.2 F.sub.4).sub.P (OCF.sub.2).sub.q                           OCF.sub.2                                                     compound 8      F(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n CF.sub.2 CF.sub.2       compound 9      F(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n CF.sub.2 CF.sub.2       compound 10     F(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n CF.sub.2 CF.sub.2       compound 11     F(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n CF.sub.2 CF.sub.2       compound 12     CF.sub.3 (OCFCF.sub.2).sub.m (OCF.sub.2).sub.1                                  CF.sub.3                                                    compound 13     CF.sub.3 (OCFCF.sub.2).sub.m (OCF.sub.2).sub.1                                  CF.sub.3                                                    compound 14     CF.sub.3 (OCFCF.sub.2).sub.m (OCF.sub.2).sub.1                                  CF.sub.3                                                    ______________________________________                                    

                  TABLE 9                                                         ______________________________________                                        compound molecular                                                            name     weight    R.sup.1      R.sup.2                                                                            R.sup.3                                                                            R.sup.4                             ______________________________________                                        compound 1                                                                             2000      C.sub.18 H.sub.37                                                                          H    H    H                                   compound 2                                                                             2000      C.sub.16 H.sub.33                                                                          H    H    H                                   compound 3                                                                             2000      C.sub.14 H.sub.29                                                                          H    H    H                                   compound 4                                                                             2000      C.sub.12 H.sub.25                                                                          H    H    H                                   compound 5                                                                             2000      C.sub.18 H.sub.35                                                                          H    H    H                                   compound 6                                                                             2000      iso-C.sub.18 H.sub.37                                                                      H    H    H                                   compound 7                                                                             4000      C.sub.18 H.sub.31                                                                          H    H    H                                   compound 8                                                                             3500      C.sub.18 H.sub.37                                                                          CH.sub.3                                                                           CH.sub.3                                                                           H                                   compound 9                                                                             3500      C.sub.6 H.sub.5 (PHENYL)                                                                   H    H    H                                   compound 10                                                                            3500      C.sub.12 H.sub.25                                                                          CH.sub.3                                                                           CH.sub.3                                                                           CH.sub.3                            compound 11                                                                            3500      C.sub.24 H.sub.49                                                                          H    H    H                                   compound 12                                                                            650       C.sub.18 H.sub.37                                                                          H    H    H                                   compound 13                                                                            650       CH.sub.2 ═CHC.sub.16 H.sub.32                                                          H    H    H                                   compound 14                                                                            650       iso-C.sub.18 H.sub.37                                                                      H    H    H                                   ______________________________________                                    

That is, Co was vapor-deposited by oblique vapor deposition on thesurface of a polyethylene terephthalate film of 14 82 m in thickness forforming a magnetic metal thin film having a film thickness of 200 nm.

A protective film, comprising a carbon film, was deposited by sputteringon the surface of the magnetic metal thin film so that a total combinedfilm thickness, including the thickness of the surface oxide layer of 5nm, amounted to 20 nm.

A solution of the compounds 1 to 14 in a solvent mixture of Freon andethanol was applied to the protective film so that a coating amount wasequal to 5 mg/m² to form a lubricant layer. The magnetic tape thusobtained was cut to a width of 8 mm to produce sample tapes 1 to 14.

For each of the sample tapes 1 to 14, the friction coefficient stilldurability and shuttle durability under the conditions of thetemperature of 25° C. and the humidity of 60%, the temperature of -5° C.and the temperature of 40° C. and the humidity of 80%, were measured.The results are shown in Tables 11 and 12.

For comparison, similar measurements were made of a magnetic tape havinga carbon film formed thereon without holding a lubricant (ComparativeExample 13), magnetic tapes making use of perfluoro polyether having aterminal carboxylic group or a perfluoro polyether having a terminalhydroxyl group, as shown in Table 10, as a lubricant (ComparativeExamples 14 to 17) and a magnetic tape not having the carbon film formedthereon and having only a lubricant layer formed thereon (ComparativeExample 18). The results are shown in Table 13.

                  TABLE 10                                                        ______________________________________                                                 perfluoro polyethers                                                 ______________________________________                                        comp. Ex. 13                                                                             --                                                                 comp. Ex. 14                                                                             HOOCCF.sub.2 (OC.sub.2 F.sub.4).sub.p (OCF.sub.2).sub.q                       OCF.sub.2 COOH                                                     comp. Ex. 15                                                                             F(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n CF.sub.2 CF.sub.2 COOH       comp. Ex. 16                                                                             CF.sub.3 (OCFCF.sub.2).sub.m (OCF.sub.2).sub.1 COOH                             CF.sub.3                                                         comp. Ex. 17                                                                             HOCH.sub.2 CF.sub.2 (OC.sub.2 F.sub.4).sub.p (OCF.sub.2).sub.q                OCF.sub.2 CH.sub.2 OH                                              comp. Ex. 18                                                                             HOOCCF.sub.2 (OC.sub.2 F.sub.4).sub.p (OCF.sub.2).sub.q                       OCF.sub.2 COOH                                                     ______________________________________                                    

                  TABLE 11                                                        ______________________________________                                                                            shuttle                                   sample                       still  durability                                table              frictional                                                                              durability                                                                           (number of                                Nos.  conditions   coefficient                                                                             (min)  times)                                    ______________________________________                                        1     25°                                                                           C., 60 RH 0.19    >720   >150                                          40°                                                                           C., 80 RH 0.24    >720   >150                                          -5°                                                                           C.        0.20    >720   >150                                    2     25°                                                                           C., 60 RH 0.20    >720   >150                                          40°                                                                           C., 80 RH 0.25    >720   >150                                          -5°                                                                           C.        0.20    >720   >150                                    3     25°                                                                           C., 60 RH 0.20    >720   >150                                          40°                                                                           C., 80 RH 0.25    >720   >150                                          -5°                                                                           C.        0.21    >720   >150                                    4     25°                                                                           C., 60 RH 0.21    >720   >150                                          40°                                                                           C., 80 RH 0.26    >720   >150                                          -5°                                                                           C.        0.21    >720   >150                                    5     25°                                                                           C., 60 RH 0.21    >720   >150                                          40°                                                                           C., 80 RH 0.26    >720   >150                                          -5°                                                                           C.        0.22    >720   >150                                    6     25°                                                                           C., 60 RH 0.20    >720   >150                                          40°                                                                           C., 80 RH 0.26    >720   >150                                          -5°                                                                           C.        0.22    >720   >150                                    7     25°                                                                           C., 60 RH 0.22    >720   >150                                          40°                                                                           C., 80 RH 0.27    >720   >150                                          -5°                                                                           C.        0.22    >720   >150                                    8     25°                                                                           C., 60 RH 0.19    >720   >150                                          40°                                                                           C., 80 RH 0.24    >720   >150                                          -5°                                                                           C.        0.19    >720   >150                                    9     25°                                                                           C., 60 RH 0.21    >720   >150                                          40°                                                                           C., 80 RH 0.27    >720   >150                                          -5°                                                                           C.        0.23    >720   >150                                    10    25°                                                                           C., 60 RH 0.19    >720   >150                                          40°                                                                           C., 80 RH 0.22    >720   >150                                          -5°                                                                           C.        0.20    >720   >150                                    ______________________________________                                    

                  TABLE 12                                                        ______________________________________                                                                            shuttle                                   sample                       still  durability                                table              frictional                                                                              durability                                                                           (number of                                Nos.  conditions   coefficient                                                                             (min)  times)                                    ______________________________________                                        11    25°                                                                           C., 60 RH 0.17    >720   >150                                          40°                                                                           C., 80 RH 0.19    >720   >150                                          -5°                                                                           C.        0.17    >720   >150                                    12    25°                                                                           C., 60 RH 0.21    >720   >150                                          40°                                                                           C., 80 RH 0.26    >720   >150                                          -5°                                                                           C.        0.23    >720   >150                                    13    25°                                                                           C., 60 RH 0.22    >720   >150                                          40°                                                                           C., 80 RH 0.28    >720   >150                                          -5°                                                                           C.        0.23    >720   >150                                    14    25°                                                                           C., 60 RH 0.23    >720   >150                                          40°                                                                           C., 80 RH 0.29    >720   >150                                          -5°                                                                           C.        0.24    >720   >150                                    ______________________________________                                    

                  TABLE 13                                                        ______________________________________                                                                          shuttle                                                              still    durability                                                 frictional                                                                              durability                                                                             (number of                                  conditions     coefficient                                                                             (min)    times)                                      ______________________________________                                        comp. 25°                                                                           C., 60 RH 0.34    >720   55                                      Ex.   40°                                                                           C., 80 RH 0.54    >720   25                                      1     -5°                                                                           C.        0.39    >720   41                                      comp. 25°                                                                           C., 60 RH 0.24    >720   125                                     Ex.   40°                                                                           C., 80 RH 0.27    391    65                                      2     -5°                                                                           C.        0.25    251    60                                      comp. 25°                                                                           C., 60 RH 0.22    >720   110                                     Ex.   40°                                                                           C., 80 RH 0.23    376    57                                      3     -5°                                                                           C.        0.22    129    48                                      comp. 25°                                                                           C., 60 RH 0.24    395    92                                      Ex.   40°                                                                           C., 80 RH 0.26    267    39                                      4     -5°                                                                           C.        0.51    120    31                                      comp. 25°                                                                           C., 60 RH 0.24    490    80                                      Ex.   40°                                                                           C., 80 RH 0.26    267    48                                      5     -5°                                                                           C.        0.25    120    41                                      comp. 25°                                                                           C., 60 RH 0.24    135    >150                                    Ex.   40°                                                                           C., 80 RH 0.27    91     55                                      6     -5°                                                                           C.        0.25    51     60                                      ______________________________________                                    

In Tables 11 to 13, still durability denotes the time involved until theoutput was decreased by -3 dB in a pause state. Shuttle durabilitydenotes the number of shuttle operations involved until the output wasdecreased by 3 dB in case of shuttle running operations each continuingfor two minutes.

It is seen from Tables 11 to 13 that low frictional coefficients, goodrunning performance and high durability could be achieved in the case ofemploying an amine salt of perfluoro polyether having a terminalcarboxylic group(s) as in the present Examples, rather than in the caseof employing solely the perfluoro polyether having a terminal carboxylicgroup(s) or the hydroxyl group, as a lubricant.

It is also seen from comparison of the sample tapes 1 to 14 and theComparative Example 18 that by forming a carbon film on the surface of amagnetic layer in addition to using the lubricant, highly satisfactoryresults could be achieved, with the frictional coefficients, stilldurability or shuttle durability not being deteriorated even underhostile environments including low temperatures or high temperature andhigh humidity.

Measurements were also made of the powder debris and decease in theshuttle output under the conditions of the temperature of 25° C. and thehumidity of 60% for the sample tapes 1 to 14 and the ComparativeExamples 13 to 18. The results are shown in Table 14. Meanwhile, thepowder debris was evaluated by observing the surface of a magnetic layerby an optical microscope, while the decrease in the shuttle output waschecked after 100 number of times of the shuttle running operations.

                  TABLE 14                                                        ______________________________________                                                powder debris                                                                            decreased shuttle output (dB)                              ______________________________________                                        sample tape 1                                                                           ∘                                                                              -0.5                                                   sample tape 2                                                                           ∘                                                                              -0.6                                                   sample tape 3                                                                           ∘                                                                              -0.8                                                   sample tape 4                                                                           ∘                                                                              -0.4                                                   sample tape 5                                                                           ∘                                                                              -0.6                                                   sample tape 6                                                                           ∘                                                                              -0.9                                                   sample tape 7                                                                           ∘                                                                              -0.7                                                   sample tape 8                                                                           ∘                                                                              -0.5                                                   sample tape 9                                                                           ∘                                                                              -0.7                                                   sample tape 10                                                                          ∘                                                                              -0.0                                                   sample tape 11                                                                          ∘                                                                              -0.6                                                   sample tape 12                                                                          ∘                                                                              -0.5                                                   sample tape 13                                                                          ∘                                                                              -0.6                                                   sample tape 14                                                                          ∘                                                                              -0.7                                                   comp. Ex. 13                                                                            x            -5.3                                                   comp. Ex. 14                                                                            x            -3.2                                                   comp. Ex. 15                                                                            x            -3.8                                                   comp. Ex. 16                                                                            x            -3.1                                                   comp. Ex. 17                                                                            x            -2.9                                                   comp. Ex. 18                                                                            x            -3.1                                                   ______________________________________                                    

It is seen from Table 14 that with sample tapes 1 to 14, powder debrisand decrease in the output occurred only to a lesser extent.

Experiment 2

On the surface of a polyethylene terephthalate base film 10 μm inthickness, a dispersion of a mixture of a binder component consistingessentially of an acrylate and SiO₂ particles having a mean particlesize of 18 nm in isopropyl alcohol as solvent was coated so that thedensity of the SiO₂ particles amounted to 10,000,000/mm² to form surfaceprotrusions.

An alloy of Co₈₀ Ni₂₀, where the subscripts denote weight percent wascharged into a crucible disposed within a vacuum chamber, and obliquevapor deposition was carried out as conventionally for depositing themagnetic material vaporized from the crucible on the surface of therunning base film for forming a thin magnetic metal film thereon. Anoxygen gas was introduced onto the surface of the base film at a rate of300 cc/min. The angle of incidence of the magnetic material on the basefilm surface was set so as to be changed in a range of 40° to 90°relative to the base film surface. The running speed of the base filmwas adjusted so that the film thickness of the produced thin magneticmetal film amounted to 200 nm.

A back coating layer consisting essentially of urethane and carbon wasformed on the opposite surface of the base film, that is its surfaceopposite to the base film surface on which the thin magnetic metal filmwas formed.

Then, using a continuous take-up type sputtering device, dc magnet tonsputtering was carried out in an Ar gas atmosphere for depositing acarbon film on the surface of the thin magnetic metal film surface.During sputtering, the vacuum was set to 2 Pa, while the tape feed ratewas adjusted so that the thickness of the carbon film amounted to 20 nm.A square-shaped target of 200 mm in width and 150 mm in length wasemployed, and a distance between the target and the substrate of themagnetic tape was set to 50 mm. The magnetic tape was cut to width of 8mm and the cut tape segments were immersed in lubricants, obtained bydissolving compound A and B shown in Table 15 in Freon 113 each at aconcentration of 0.06 wt %, at a rate of 1 m/min, for forming alubricant layer on the surface of the carbon film, for producing sampletapes 15 and 16.

                                      TABLE 15                                    __________________________________________________________________________    lubricants                                                                           structural formula                                                     __________________________________________________________________________    compound A                                                                           H.sub.37 C.sub.18 H.sub.3 N.sup.+.O.sup.- OCCF.sub.2 O(CF.sub.2               O).sub.n (C.sub.2 F.sub.4 O).sub.m CF.sub.2 COO.sup.-..sup.+                  NH.sub.3 C.sub.18 H.sub.37                                             compound B                                                                           F(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n CF.sub.2 CF.sub.2 COO.sup.-..           sup.+ NH.sub.3 C.sub.18 H.sub.37                                       compound C                                                                           F(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n CF.sub.2 CF.sub.2 COO.sup.-..           sup.+ NH.sub.3 C.sub.18 H.sub.37                                       compound D                                                                           F(CF.sub.2 CF.sub.2 CF.sub.2 O).sub.n CF.sub.2 CF.sub.2 COO.sup.-..           sup.+ NH(CH.sub.3).sub.2 C.sub.18 H.sub.37                             __________________________________________________________________________

Of the sample tapes 15 and 16, prepared in this manner, shuttledurability, shuttle durability and partial proceeding wear were checked.The results are shown in Table 16. Still durability was checked bystill-running the sample tapes at ambient temperature and humidity untiltwo hours have elapsed, using a remodelled version of a video deckmanufactured by SONY CORPORATION under a trade name of EV-Si, andchecking the time which elapsed until the output was decreased by -3 dB.Shuttle durability was evaluated in terms of a decrease in the outputafter 100 passes with respect to the output after 1 pass at an ambienttemperature and humidity using the video deck manufactured by SONYCORPORATION under the trade name of EV-S1. This shuttle durability isrepresented by the following equation (2): ##EQU2##

On the other hand, partial proceeding wear was evaluated by observingstep differences produced on forming interference fringes by an opticalmicroscope.

                  TABLE 16                                                        ______________________________________                                                      still     shuttle   partially                                                 durability                                                                              durability                                                                              proceeding                                  lubricant     (min)     (dB)      wear                                        ______________________________________                                        sample compound A >1400     0       ∘                             tape 15                                                                       sample compound B 20˜30                                                                             -2      Δ                                   tape 16                                                                       ______________________________________                                    

It is seen from Table 16 that by using an amine salt of perfluoropolyether having a carboxylic group at one or both terminals as alubricant partially proceeding wear could be diminished in either cases,so that satisfactory durability could be achieved.

Experiment 3

On the surface of a polyethylene terephthalate film, having a thicknessof 10 μm, Co₈₀ Ni₂₀, wherein subscripts denote wt % values, wasdeposited by vacuum deposition to form a thin magnetic metal film. Anoxygen gas was introduced onto the base film surface at a rate of 2500cc/min, and the angle of incidence of the magnetic material relative tothe base film surface was set to 45°.

Sputtering was then carried out in an Ar gas atmosphere for depositing acarbon film on the thin magnetic metal film surface to a film thicknessof 20 nm. During sputtering, the Ar gas flow rate was set to 300 SCCM,while the feed rate of the magnetic tape was set to 1 m/min.

The magnetic tape was immersed in lubricants, obtained by dissolvingcompounds C and D shown in Table 9 in Freon 113 each at a concentrationof 0.06 wt %, at a rate of 1 m/min, for forming a lubricant layer on thesurface of the carbon film, for producing sample tapes 17 and 18.

of these sample tapes 17 and 18, prepared in this manner, stilldurability, shuttle durability, surface roughness and powder debris werechecked. The results are shown in Table 17. Still durability was checkedby still-running the sample tapes using a video deck manufactured bySONY CORPORATION under a trade name of EV-Si and checking the time whichelapsed until the output was decreased by -3 dB. Shuttle durability wasevaluated in terms of a decrease in the output after 100 passes ofshuttle running operations, each continuing for two minutes, using thevideo deck manufactured by SONY CORPORATION under the trade name ofEV-S1. Surface roughness was measured using a talistep with a needlepressure of 2 mg/mm², while powder debris was evaluated by observing themagnetic head surface by an optical microscope (magnification factor:100) after using the sample tapes 17 and 18 kept in sliding contact withthe magnetic head.

For comparison, a magnetic tape in which a commercial perfluoropolyether (trade name: Z-DOL) was used in place of the compound C (orcompound D) was checked in a similar manner. The results are also shownin Table 17.

                  TABLE 17                                                        ______________________________________                                                     still   shuttle                                                               dura-   dura-   surface                                                       bility  bility  roughness                                                                              powder                                  lubricant    (hrs)   (dB)    (Å)  debris                                  ______________________________________                                        sample compound  20      +0.3  20                                             tape 17                                                                              C                                                                      sample compound  15      -0.1  20       ∘                         tape 18                                                                              D                                                                      comp.  Z-DOL     2       -4.8  20       x                                     Ex. 19                                                                        ______________________________________                                    

It has been found that excellent durability and only insignificantpowder debris may be achieved by employing amine salt compounds ofperfluoro polyether, having terminal carboxylic group(s), as a lubricantas shown in Table 17.

Thus, the effects on Various properties caused by changes in themolecular weights of the perfluoro polyether derivatives employed as alubricant were checked.

Experiment 4

Sample tapes 19 to 22 were prepared by sequentially stacking a magneticmetal thin film and a carbon film on a base film similarly to the aboveExperiment 3 and subsequently forming a lubricant layer on the carbonfilm surface using a variety of perfluoro polyether derivatives havingdifferent molecular weights under conditions similar to those employedin Experiment 3. Meanwhile, the perfluoro polyether derivatives employedin the present experiment had molecular weights as shown in Table 18.

The shuttle durability, surface roughness and the frictional coefficientof the sample tapes 19 to 22 were measured similarly to Experiment 3.The results are also shown in Table 18.

                  TABLE 18                                                        ______________________________________                                               molecu-                                                                              shuttle  surface  pow-  fictional                                      lar    durabil- rough-   der   coef-                                          weight ity(db)  ness(Å)                                                                            debris                                                                              ficient                                 ______________________________________                                        sample   1400     +0.4     20     ⊚                                                                    0.20                                  tape 19                                                                       sample   2500     -0.5     20     ∘                                                                       0.19                                  tape 20                                                                       sample   3500     -1.0     20     ∘                                                                       0.21                                  tape 21                                                                       sample   4500     -1.2     20     ∘                                                                       0.22                                  tape 22                                                                       comp. Ex. 20                                                                           1000     -2.5     20     x     0.27                                  comp. Ex. 21                                                                           5000     -3.5     20     x     0.29                                  ______________________________________                                    

As shown in Table 18, the frictional coefficient could be suppressedsufficiently, while excellent durability could be achieved and powderdebris could be prevented from being incurred when the molecular weightof the perfluoro polyether derivative employed as a lubricant was in arange of from 1400 to 4500. Conversely, if the molecular weight of theperfluoro polyether derivative was lesser than the above range(Comparative Example 20) or larger than the above range (ComparativeExample 21), the decrease in the output after repeated running waspredominant, while powder debris was increased.

The relation bet ween various properties and the molecular weight s ofpolar group moiety of the above-mentioned perfluoro polyetherderivatives was then investigated.

Experiment 5

Sample tapes 23 to 25 were prepared by sequentially stacking a magneticmetal thin film and a carbon film on a base film similarly to the aboveExperiment 2 and subsequently forming a lubricant layer on the carbonfilm surface using three different perfluoro polyether derivativeshaving different polar groups under the same conditions as those ofExperiment 2. Meanwhile, the perfluoro polyether derivatives had polargroups on both terminals, while the structure and the molecular weightof the polar group moieties had the structure and the molecular weightas shown in Table 19.

The shuttle durability and partially advancing wear of the sample tapes23 to 25 were measured similarly to Experiment 2. The results are shownin Table 12. In the present Experiment, a remodelled video deckmanufactured by SONY CORPORATION under the trade name of EV-S1 wasemployed for conducting e shuttle durability test in the presentExperiment.

For comparison, measurements were also made of the cases of employingperfluoro polyether derivative having the molecular weight of the polargroup moiety exceeding 120 (Comparative Examples 22 and 23). The resultsare also shown in Table 19.

                  TABLE 19                                                        ______________________________________                                        structure of              shuttle  partially                                  polar group     molecular durability                                                                             proceeding                                 moiety          weight    (dB)     wear                                       ______________________________________                                        sample                                                                              --COONH.sub.2 (CH.sub.3)                                                                    75        -0.5   ∘                            tape 23                                                                       sample                                                                              --COONH(CH.sub.3).sub.2                                                                     89        -0.4   ∘                            tape 24                                                                       sample                                                                              --COONH(C.sub.2 H.sub.5).sub.2                                                              117       -0.7   Δ                                  tape 25                                                                       comp. --COONH(C.sub.3 H.sub.7).sub.2                                                              145       -1.4   x                                        Ex. 22                                                                        comp. --COON(C.sub.3 H.sub.7).sub.3                                                               187       -1.8   xx                                       Ex. 23                                                                        ______________________________________                                    

The tendency is noticed that the larger the molecular weight of thepolar group moieties, the more pronounced is the deterioration in theoutput after repeated running, as shown in Table 19. Similar resultshave been obtained with partially advancing wear. It is seen from thisthat the molecular weight of the polar group moiety needs to be lowerthan 120 for assuring satisfactory durability.

Scrutinies into Surface Properties of Reverse Surface of non-magneticBase film

A vacuum deposition device for producing a magnetic recording medium ofthe present embodiment is hereinafter explained.

This vacuum deposition device is employed for forming a magnetic layerby vacuum deposition of two magnetic metal thin films, and includes afeed-out roll 583 and a take-up roll 583 at the upper left and rightsides within a vacuum chamber 581, the internal region of which ismaintained at a reduced pressure, and first and second cooling cans 584and 585 at the central left and right sides of the vacuum chamber 581,as shown in FIG. 15. A guide roll 586 is provided between the feed-outroll 582 and the cooling can 584, while guide rolls 587, 588 areprovided between the cooling cans 584 and 585 and a guide roll 589 isprovided between the cooling can 585 and the take-up roll 583.

The first cooling can 584 is mounted for reeling out the flexiblesupporting base film 590, wound about the feed-out roll 582, downwardsin the drawing, while the second cooling can 585 is provided at the sameheight as the cooling can 584. The cooling cans 584, 585 are larger indiameter than the feed-out roll 582 or take-up roll 583. The feed-outroll 582, take-up roll 583 and the cooling cans 584, 585 are in the formof cylinders each having a width substantially equal to that of theflexible supporting base film 590. The cooling cans 584, 585 areprovided with internal cooling means, not shown, for preventing theflexible supporting base film from being deformed due to rise intemperature.

The guide rolls 586, 589 are provided between the feed-out roll 582 andthe cooling can 584 and between the cooling can 585 and the take-up roll583, respectively, while the other guide rolls 587, 588 are provided ata height above and between the first cooling can 584 and the secondcooling can 585, respectively. Thus the guide roll 586, 587, 588 and 589apply a predetermined tension to the flexible supporting base film 590,which is being supplied from the feed-out roll 582 to the first coolingcan 584 and thence to the second cooling can 585 so as to be taken upfrom the second cooling can 585 on the take-up roll 583, so as to permitthe flexible supporting base film 590 to be run positively on theperipheral surfaces of the cooling cans 584, 585.

A pair of crucibles 591, 592, filled with a magnetic metal material, areprovided below the cooling cans 584, 585. A shutter 594, curved toconform to the peripheral surface of the cooling can 584, is provided inthe vicinity of the cooling can 584 intermediate between the firstcooling can 584 and the crucible 591 filled with the magnetic metalmaterial 593. Similarly, another shutter 595 is mounted in associationwith the second cooling can 585. A monitoring window 596 is provided ina wall surface of the vacuum chamber 581 laterally of the cooling can585, while heating units 597, 598 are provided outside of the vacuumchamber 581.

In the above-described vacuum deposition device, the flexible supportingbase film 590, wound around the feed-out roll 582, is supplied from thefeed-out roll 582, rotated counterclockwise at a constant speed, to thefirst cooling can 584, rotated clockwise at a constant speed, while thebase film is tensioned by guide roll 586. The base film is then guidedupwards in the drawing so as to be run around the guide roll 588, whilebeing tensioned by the guide roll 587. The flexible supporting base filmis then supplied from guide roll 588 to the second cooling can 585,rotated clockwise at a constant speed, so as to be taken up by take-uproll 583, rotated clockwise at a constant speed, while being tensionedby guide roll 589.

Vapor deposition is performed on the non-magnetic base film 590 as it isrun along the peripheral surfaces of the first cooling can 584 and thesecond cooling can 585.

Specifically, the aforementioned crucible 591 filled with the magneticmetal material 593 is provided below the first cooling can 584 withinthe vacuum chamber shown in FIG. 15, and has a width equal to that ofthe first cooling can 584. The magnetic metal material 593 charged intothe crucible 591 is heated and evaporated by heating unit 597 providedexternally of the vacuum chamber 581. The magnetic metal material 593,evaporated by the heating unit 597, is deposited as a first magneticmetal thin film on the flexible supporting base film 590 which is run ata constant speed around the peripheral surface of the cooling can 584.

The flexible supporting base film 590, having the first magnetic metalthin film deposited thereon, is guided upwards in the drawing by guideroll 587 and thence to guide roll 588 so as to be supplied to the secondcooling can 585. The second cooling can 585 is associated with thecrucible 592 similarly to the first cooling can 584. The magnetic metalmaterial 593, charged in the crucible 592, is heated and evaporated bythe heating unit 598 provided externally of the vacuum chamber 581. Thusthe magnetic metal material 593, evaporated by the heating unit 598, isdeposited as a second magnetic metal thin film on the flexiblesupporting base film 590, having the first magnetic metal thin filmdeposited thereon and which is run at a constant speed around theperipheral surface of the cooling can 585.

It is noted that, since the aforementioned shutters 594, 595 areprovided to cover predetermined portions of the cooling cans 584, 585,respectively, the magnetic metal material 593 is allowed to be depositedwithin a range covered by a predetermined angle θ to form the first andsecond magnetic metal thin films.

The magnetic recording medium, prepared by the above-described device,has a structure as shown in FIG. 16, in which a first magnetic metalthin film 601a and a second magnetic metal thin film 601b, making up amagnetic layer 601, are formed on one of the major surfaces of aflexible supporting base film 599 formed e.g. of polyethyleneterephthalate.

A first undercoat layer 600 is coated on the major surface of theflexible supporting base film 599, with the first magnetic metal thinfilm 601a being formed via the undercoat layer 600 on the flexiblesupporting base film 599. The opposite major surface of the flexiblesupporting base film 599 is formed with numerous protrusions 599a causedby the presence of fillers 602. A protective film 603 is formed on themagnetic layer 601, while a backcoat 604 is formed on the opposite majorsurface of the flexible supporting base film 599.

Using the above-described vacuum deposition device, a variety ofmagnetic tapes, each having a magnetic layer constituted by theabove-described two magnetic metal thin films, were prepared.

The undercoat films, magnetic films, back coats and the top coats wereformed on polyethylene flexible supporting base films, each being 10 μmin thickness and having an extremely smooth major surface (evaporatedsurface) and the other major surface (running surface) presentingvariable roughnesses, by way of preparing magnetic tapes of ComparativeExamples 24 to 27 and Examples 26 to 28.

The same conditions were used for preparing the undercoat films, thatis, an acrylate based emulsion, having a mean particle size of 250 Å,was coated at a rate of 1,000,000 particles per mm² on the evaporatedsurface, for forming each undercoat layer.

The same vapor deposition conditions were used, that is, the same Co₉₅--Ni₅, where subscripted numbers stand for wt % values, was used, whilethe amount of introduced oxygen, the temperature for the first andsecond cooling cans, the incident angle and the tape speed were set to200 cc/min, -20° C., 45° to 90° and to 18 m/min, respectively. Themagnetic layer was formed with the thicknesses of the first and secondmagnetic metal thin films both being set to 1000 Å.

After vapor deposition of the magnetic layer, a back coat composedmainly of a carbon pigment and an urethane-based binder was applied, andthe resulting product was processed with H/P at 150° C. for 0.3 sec andtop coating and cut to predetermined tape widths to prepare magnetictapes of the Comparative Examples 24 to 27 and Examples 26 to 28.

The number of the occurrences of thermal degradations, surface roughnessof the magnetic layer, energy products and the error rate of thesesample tapes during preparation thereof were measured. The number ofoccurrences of thermal degradations was checked by visual observationthrough the monitoring window 516 shown in FIG. 15 directly afterinitial formation of a coil of the second magnetic metal thin film onthe peripheral surface of the second cooling can, and represents thenumber encountered in the course of preparation of a 1000 m length ofeach of the sample tapes of the Comparative Examples 24 to 27 andExamples 26 to 28.

The surface roughnesses were measured for a scan length of 0.5 mm usinga talistep prepared by Lank Taylor Bobson Inc. and a trapezoidal needlehaving a needle diameter of 0.2×0.2 μm. The energy product was found asa product of the residual magnetic flux density B_(r), saturationmagnetization δ and coercivity H_(c). The results are shown in Table 20.

                                      TABLE 20                                    __________________________________________________________________________    surface             surface                                                   roughness of number of                                                                            roughness                                                 running      occurrences                                                                          of magnetic                                                                         energy                                              surface (μm)                                                                            of thermal                                                                           layer product                                                                             error                                         R.sub.a   R.sub.max                                                                        deterioration                                                                        (μm)                                                                             (G cm Oe)                                                                           rate                                          __________________________________________________________________________    comp.                                                                             24                                                                              0.0065                                                                            0.078                                                                            7      0.0025                                                                              102   1.8 × 10.sup.-4                         Ex. 25                                                                              0.0073                                                                            0.068                                                                            8      0.0029                                                                              105   2.1 × 10.sup.-4                             26                                                                              0.0012                                                                            0.002                                                                            --     --    --                                                      27                                                                              0.0016                                                                            0.014                                                                            --     --    --                                                  Ex. 26                                                                              0.0063                                                                            0.065                                                                            0      0.0020                                                                              110   7.8 × 10.sup.-5                             27                                                                              0.0046                                                                            0.034                                                                            0      0.0015                                                                              111   6.5 × 10.sup.-5                             28                                                                              0.0052                                                                            0.042                                                                            0      0.0018                                                                              108   6.8 × 10.sup.-5                         __________________________________________________________________________

It is seen from Table 20 that with the Examples 26 to 28 in which thecenterline mean roughness R_(a) and maximum height of the projectionsR_(max), as measured of the roughness of the running surface of theflexible supporting base film, are within the ranges of the presentinvention, as contrasted to Comparative Examples 24 too 27 in whichR_(a) or R_(max) are outside the range of the present invention.Besides, thermal degradation and hence the roughness of the magneticlayer side of the sample tapes are suppressed to assure a smooth surfaceof the magnetic layer, and both the energy product and error rate areimproved. With the Comparative Examples 26 and 27, wrinkles wereproduced on the surface during tape running along the cooling can torender it impossible to make measurements of the parameters in theirentirety.

Meanwhile, the above-given definition of the surface roughness of thereverse surface may advantageously be employed for the cases in whichthe protective layer is formed via an oxide layer or a bombardmentoperation is carried out as in the preceding Examples.

Constitution of Recording/Reproducing Apparatus

The magnetic recording medium of each of the previous embodiments may beemployed advantageously as a recording medium for a digital VTR.

The digital VTR is used for recording digitized color video signals on arecording medium, such as a magnetic tape. The component type digitalVTR of a D1 format for broadcasting stations and a composite typedigital VTR of a D2 format have been put to practical use.

With the former type D1 format digital VTR, luminance signal and firstand second chrominance signals are processed by analog/digitalconversion at the sampling frequencies of 13.5 MHz and 6.75 MHz,respectively, followed by predetermined signal processing, and theresulting signal are recorded on a magnetic tape. The D1 format digitalVTR system is also termed a 4:2:2 system in that the samplingfrequencies of these signal components bear a ratio of 4:2:2.

With the later D2 format digital VTR, the composite color video signalsare sampled by signals having a frequency four times that of the colorsubcarrier signals and processed with analog/digital conversion andpredetermined signal processing before being recorded on a magnetictape.

At any rate, since these digital VTRs are designed on the premises thatthey are destined for being used at the broadcasting stations, specialemphasis is placed on the picture quality and digital color signalsconverted by A/D at a rate of 8 bits per sample are recorded withoutsubstantial data compression.

Consequently, with e.g. the D1 format digital VTR, the playback time of1.5 hour at most may be achieved when using a large-sized cassette tape,so that it cannot be suitably employed as a VTR for general householdapplication.

Thus, in the above-described embodiment of the digital VTR, signalshaving a minimum wavelength of 0.5 μm are recorded for e.g. a trackwidth of 5 μm for realizing a recording density of 8×10⁵ bits/mm².Besides, a data compression system of compressing the recording data ina manner free from playback distortion is simultaneously employed forenabling a narrow tape with a tape width of 8 mm or less to be used forrecording/playback continuing for an extended period of time. Thearrangement of this digital VTR is hereinafter explained.

a. Signal processing Unit

First the signal processing unit of the digital VTR employed in thepresent embodiment is explained.

FIG. 17 shows an arrangement of the recording side in its entirety.Digital luminance signals Y and digital color difference signals U, V,prepared from three color signals R, G and B of a color video camera,are supplied to input terminals Y, IU and IV. The clock rates of therespective signals are selected to be equal to the frequencies of thecomponent signals of the D1 format. That is, the sampling frequenciesare set to 13.5 MHz and 6.75 MHz, with the number of bits per samplebeing 8. Thus the data volume of the signals supplied to the inputterminals IY, IU and IV is approximately 216 Mbps. These signals arecompressed to a data volume of approximately 167 Mbps by being passedthrough an effective data extracting circuit 2 adapted to foreliminating blanking data from the signals.

Among the outputs of the effective data extracting circuit 2, theluminance signals Y are supplied to a frequency converter 3 where thesampling frequency is reduced to three fourths of 13.5 MHz. Asub-sampling filter, for example, is used as the frequency converter 3to prevent aliasing distortion. An output signal from the frequencyconverter 3 is supplied to a block-forming circuit 5 where the sequenceof the luminance data is changed in a block sequence. The block-formingcircuit 5 is provided for a downstream block-forming and encodingcircuit 8.

FIG. 19 shows a structure of a block as an encoding unit. The exampleshown refers to a three-dimensional block. That is, a number of unitblocks each consisting of 4 lines×4 pixels×2 frames as shown in FIG. 19are formed by dividing a picture across two frames. In FIG. 19, a solidline and a broken line indicate horizontal scanning lines for anodd-numbered field and even-numbered field, respectively.

Among the outputs of the effective data extracting circuit 2, the twocolor difference signals U and V are supplied to a sub-sampling andsub-line circuit 4 where the sampling frequency is halved from 6.75 MHzand subsequently the two digital color difference signals arealternately selected from line to line so as to be synthesized to a1-channel data. Consequently, line-sequential digital color differencesignals are produced from the sub-sampling and sub-line circuit 4. FIG.20 shows a pixel array of signal s sub-sampled and sub-lined by thesub-sampling and sub-line circuit 4. In FIG. 20, ∘, Δ and × denotesub-sampled pixels of the first color difference signals U, sub-sampledpixels of the second color difference signals V and the positions of thepixels eliminated by sub-sampling, respectively.

The line-sequential output signals from the sub-sampling and sub-liningcircuit 4 are supplied to a block-forming circuit 6. Similarly to theblock-forming circuit 5, the block-forming circuit 6 converts the colordifference data in the television signal scanning sequence convertedinto block sequence data. Similarly to the block-forming circuit 5, theblock-forming circuit 6 converts the color difference data into datahaving a block structure of 4 lines×4 pixels×2 frames. Output signals ofthe block-forming circuits 5 and 6 are supplied to a synthesis circuit7.

In the synthesis circuit 7, the luminance signals and color differencesignals, converted into block sequence data, are converted intoone-channel data, which is supplied to a block-forming encoding circuit8. The block-forming encoding circuit 8 may for example be an adaptiveblock-by-block dynamic range coding, (ADRC) circuit as later explained,or a discrete cosine transform (DCT) circuit. An output signal from theblock-forming encoding circuit 8 is supplied to a frame-forming circuit9 where the signal is converted into frame data. The frame-formingcircuit 9 provides for switching from clocks for an pixel system toclocks for a recording system.

An output signal of the frame-forming circuit 9 is supplied to a circuit10 for generating parities as error correction code data. An outputsignal of the parity generating circuit 10 is supplied to a channelencoder 11 where channel encoding is performed for reducinglow-frequency components of the recording data. An output signal ofchannel encoder 11 is supplied to each of the recording amplifiers 12Aand 12B and to a rotary transformer, not shown, and thence to a pair ofmagnetic heads 13A, 13B for recording on a magnetic tape. It is notedthat audio signals and video signals are compression-encoded separatelyand supplied to channel encoder 11.

By extracting only the effective scanning periods of the input data, theinput data volume of 26 Mbps are reduced by the above signal processingto approximately 167 Mbps, which is further reduced by frequencyconversion, sub-sampling and sub-lining into 84 Mbps. These data arecompression-encoded by the block-forming and encoding circuit 8 andthereby reduced to approximately 25 Mbps. Subsidiary data such as parityand audio signals are subsequently annexed to the compressed data sothat the recording data volume becomes 31.56 Mbps.

The arrangement of a playback side is explained by referring to FIG. 18.

For playback, playback data from magnetic heads 13A, 13B are suppliedvia rotary transformer and playback amplifiers 14A, 14B to a channeldecoder 15 in which a demodulation which is the reverse of the channelencoding is carried out. An output signal of channel decoder 15 issupplied to a time base correcting circuit (TBC circuit) 16 in which theplayback signals are freed of jitter components. Playback data from TBCcircuit 16 are supplied to an ECC circuit 17 where error concealment anderror correction are carried out using error correction code data. Anoutput signal of the ECC circuit 17 is supplied to a de-framing (framedecomposition) circuit 18.

In the frame decomposition circuit 18, the respective components of theencoded block data are separated from one another, while switching fromthe clocks of the recording system to clocks of the pixel system iscarried out. The separated data from the frame decomposition circuit 18are supplied to a block decoding circuit 19, where decoded datacorresponding to original data are decoded on the block-by-block basisand the decoded data is supplied to a distribution circuit 20. Thedecoded data is separated by the distribution circuit 20 into luminancesignals and color difference signals which are supplied to blockdecomposition circuits 21, 22, respectively. The block decompositioncircuits 21, 22 perform the operation which is the reverse of theoperation performed by the transmitting side block-forming circuits 5and 6, that is, convert the decoded data in the block sequence intodecoded data in the raster scanning sequence.

On the other hand, the digital color difference signals from the blockdecomposition circuit 22 are supplied to distribution circuit 24 wherethe line-sequential digital color difference signals U and V areseparated into digital color difference signals U and V, respectively.These color difference signals U V are supplied to an interpolatingcircuit 25 and thereby interpolated. The interpolating circuit 25interpolates the data of the pixels and lines eliminated bysub-sampling, using the restored pixel data. The digital colordifference signals U and V, having a sampling rate of 2 fs, are producedby the interpolating circuit 25 so as to be outputted at outputterminals 26U, 26V, respectively.

b. Block-Forming and Encoding

As the block-forming and encoding circuit 8, shown in FIG. 17, anadaptive dynamic range coding (ADRC) encoder is employed. With the ADRCencoder, a maximum value MAX and a minimum value MIN of plural pixeldata contained in each block are detected, a dynamic range DR of eachblock is detected from the detected maximum and minimum values MAX, MIN,and encoding adapted to the dynamic range DR is carried out to achievere-quantization using the number of bits smaller than that of theoriginal pixel data. As another example of the block-forming andencoding circuit 8, it is also possible to discrete cosine transform thepixel data of each block, quantize the coefficient data resulting fromDCT and to encode the quantized data by run-length Huffman encoding byway of compression coding.

An example of an ADRC encoder in which deterioration in the picturequality is unlikely to be caused on dubbing a number of times ishereinafter explained by referring to FIG. 21.

Referring to FIG. 21, digital video signals or digital color differencesignals, quantized at a rate of 8 bits per sample, are supplied from asynthesis circuit 7 of FIG. 17 to an input terminal 27. The block datafrom input terminal 27 are supplied to a maximum value and minimum valuedetection circuit 29 and a delay circuit 30. The maximum value andminimum value detection circuit 29 detects a minimum value MIN and amaximum value MAX from block to block. The delay circuit 30 delays inputdata a time necessary for detecting the maximum and minimum values.Pixel data from delay circuit 30 are supplied to comparators 31, 32.

The maximum value MAX and the minimum value MIN from the maximum valueand minimum value detection circuit 29 are supplied to a subtractioncircuit 33 and an addition circuit 34, respectively. The subtractioncircuit 33 and the addition circuit 34 are supplied from a bit-shiftingcircuit 35 with a value of a quantization step width (Δ=1/16 DR) in caseof non-edge matching encoding with a fixed 4-bit length. Thebit-shifting circuit 35 is designed to shift the dynamic range DR byfour bits to carry out a division of 1/16. A threshold value of (MAX-Δ)is obtained from subtraction circuit 33, while a threshold value of(MAX+Δ) is obtained from addition circuit 34. These threshold valuesfrom the subtraction circuit 33 and the addition circuit 34 are suppliedto comparators 31, 32, respectively. Meanwhile, the value Δ defining thethreshold values may also be a fixed value corresponding to a noiselevel, instead of being a quantization step width.

An output signal from comparator 31 is supplied to an AND gate 36, whilean output signal from comparator 32 is supplied to an AND gate 37. TheAND gates 36, 37 are also supplied with input data from delay circuit30. The output signal of the comparator 31 goes high when the input datais larger than the threshold value, so that pixel data of the input datacomprised within a maximum range of (MIN˜MIN-Δ)is extracted at outputterminal of AND gate 36. On the other hands, the output signal of thecomparator 32 goes high when the input data is smaller than thethreshold value, so that pixel data of the input data comprised within aminimum range of (MIN˜MIN+Δ) is extracted at output terminal of AND gate37.

An output signal of AND gate 36 is supplied to an averaging circuit 38,while an output signal of AND gate 37 is supplied to an averagingcircuit 39. These averaging circuits 36, 37 each compute a mean valuefrom block to block and are supplied with a reset signal of a periodsignal to a block period from terminal 40. A mean value MAX' of thepixel data comprised within a maximum level range of (MAX˜MAX-Δ) isobtained from averaging circuit 38, while a mean value MIN' of the pixeldata comprised within a minimum level range of (MIN˜MIN+Δ) is obtainedfrom averaging circuit 39. The mean value MIN' is subtracted bysubtraction circuit 41 from the mean value MAX' so that a dynamic rangeDR' is obtained by the subtraction circuit 41.

The mean value MIN' is supplied to a subtraction circuit 42 so as to besubtracted from the input data transmitted via delay circuit 43 toproduce data PDI freed of the minimum value. The data PDI and theconcealed dynamic range DR' are supplied to a quantization circuit 44.In the present embodiment a variable length ADRC is employed in whichthe numbers of bits allocated to quantization are set to one of 0 (nottransmitting code data), 1, 2, 3 or 4, and edge matching quantization iscarried out. The number of bit allocation n is set by a bit numberdecision circuit 45 from block to block, and data of a bit number n istransmitted to quantization circuit 44.

It is possible with variable length ADRC to achieve high efficiencyencoding by allocating a smaller number of bits for blocks having asmall dynamic range DR' and by allocating a larger number of bits to ablock having a larger dynamic range DR'. That is, if the threshold valuefor deciding the number of bits n is set to T1˜T4 (T1<T2<T3<T4), no codesignal is transmitted for the block (DR'<T1) and only data of thedynamic range DR' is transmitted. For blocks of (T1<DR'<T2),(T2<DR'<T3), (T3<DR'<T4) and (DR'>T4), the numbers of bits n are (n=1),(n=2), (n=3) and (n=4), respectively.

With the above-described variable length ADRC, the amount of thegenerated information may be controlled by changing the threshold valuesT1 to T4, by way of so-called buffering. For this reason, the variablelength ADRC may be applied to a transmission channel, such as a videotape recorder of the present invention, in which the amount of theinformation generated per field or frame needs to be of a predeterminedvalue.

A plurality of, e.g. 32, sets of the threshold values (T1, T2, T3 andT4) are stored in the buffering circuit 46 which is adapted for decidingthe threshold values T1 to T4 employed for setting the amount of thegenerated information to a predetermined value. These sets of thethreshold values are distinguished by parametric codes Pi (i=0, 1, 2, .. . 31). The numbers i of the parametric codes Pi are set so that thelarger the number i, the more pronounced is the tendency for the amountof the generated information to be decreased monotonously. However, therestored picture is degraded in picture quality with decrease in theamount of the generated information.

The threshold values T1 to T4 are supplied from buffering circuit 46 toa comparator 47, while the dynamic range DR' via delay circuit 48 issupplied to comparator 47. The delay circuit 48 delays the dynamic rangeDR' by a time necessary for the set of the threshold values to bedetermined by the buffering circuit 46. The comparator 47 compares thedynamic range DR' of the block with the respective threshold values andoutputs a comparison output to a bit number decision circuit 45 wherethe number of bit allocation n for the block is set.

In the quantization circuit 44, the data PDI from delay circuit 49,freed of the minimum value data, is converted into code signals DT byedge matching quantization, using the dynamic range DR' and the numberof bit allocation n. The quantization circuit 44 is constituted by e.g.a ROM.

The concealed dynamic range data DR' and the mean value MIN' areoutputted via delay circuits 48, 50, respectively. In addition, theparametric code Pi indicating a set of threshold values and code signalDT is also outputted. In the present embodiment deterioration in picturequality on dubbing is minimized because signals once quantized bynon-edge matching quantization is quantized by edge matchingquantization based on the dynamic range information.

c. Channel Encoder and Channel Decoder

The channel encoder 11 and the channel decoder 15 shown in FIG. 17 arehereinafter explained.

In the channel error 11, as shown in FIG. 22, plural adaptive M-seriesscrambling circuits 51 are provided in an adaptive scrambling circuitsupplied with an output of the parity generating circuit 10, and such anM-series is selected for which an output with the minimum content in thehigh frequency components and in the dc components are obtained for theinput signals. A processing of 1/1-D², D being a unit delaying circuit,is carried out in a pre-coder 52 for a partial response class 4detection system. An output of the pre-coder 52 is recorded andreproduced by magnetic heads 13A, 13B via recording amplifiers 13A, 13Band the resulting playback output is amplified by playback amplifiers14A, 14B.

In the channel decoder 15, as shown in FIG. 23, a processing of 1+D isperformed on an output of the playback amplifiers 14A, 14B by a partialresponse class 4 playback processing circuit 53. At a so-calledbit-by-bit decoding circuit 54, decoding of data strong against noise iscarried out on an output of the processing circuit 53 by processingusing data correlation and plausibility processing. An output of thebit-by-bit decoding circuit 54 is supplied to a descrambling circuit 55whereby the data re-arrayed by scrambling at the recording side isrestored to the original array for restoring the original data. With thebit-by-bit decoding circuit 54, playback C/N conversion is improved by 3dB as compared to the case in which bit-by-bit decoding is to beperformed.

d. Running System

The magnetic heads 13A and 13B are attached to a drum 76 as a unifiedstructure to a drum 76, as shown in FIG. 24.

A magnetic tape, not shown, is wrapped at a wrapping angle slightlylarger or smaller than 180° on the peripheral surface of the drum 76, sothat the magnetic tape is scanned by the magnetic heads 13A and 13Bsimultaneously.

The magnetic gaps of the magnetic heads 13A and 13B are set so as to beinclined in opposite directions, that is, in such a manner that themagnetic head 13A and the magnetic head 13B are inclined by +20° and by-20° relative to the track width direction, respectively. This minimizesthe amount of cross-talk between adjacent tracks caused by so-calledazimuth losses during playback.

FIGS. 25 and 26 show a more concrete unitary structure of the magneticheads 13A, 13B in the form of a so-called double-azimuth head. Forexample, unitary magnetic heads 13A, 13B are mounted on an upper drum 76rotated at an elevated velocity, with a lower drum 77 remainingstationary. The wrapping angle θ of a magnetic tape 78 is 166°, whilethe drum diameter φ is 16.5 mm.

Consequently, one-field data are recorded in five tracks. By thissegment system, track length may be reduced for reducing errors causedby linear characteristics of the tracks.

By simultaneous recording by the double-azimuth magnetic heads, itbecomes possible to reduce errors caused by linearity as compared to thecase in which a pair of magnetic heads are placed at a diametricallyopposite positions on the drum. On the other hand, bearing adjustmentmay be made more accurately because of a limited distance between theheads. Consequently, recording/playback may be achieved with a track ofa narrower width.

What is claimed is:
 1. A magnetic recording medium comprising:a basefilm having first and second opposed surfaces, said first surface havinga centerline mean roughness, R_(a), of greater than or equal to 0.0015μm and less than equal to 0.0070 μm and having a maximum protrusionheight R_(max) of greater than or equal to 0.0015 μm and less than orequal to 0.070 μm; a magnetic metal thin film layer disposed on saidsecond surface of the base film; an oxide layer having a thickness offrom about 20 to about 230 Å disposed on said magnetic metal thin film;a protective layer having a thickness of from about 20 to about 230 Ådisposed on said oxide layer, the combined total thickness of said oxidelayer and said protective layer being from about 40 to 250 Å; and alubricant layer disposed on said protective layer, said lubricant layerincluding a perfluoro polyether derivative formed by reaction of acarboxy-terminated perfluoro polyether with a hydrocarbon substitutedquaternary ammonium compound and selected from derivatives having theformula: ##STR6## wherein Rf represents a perfluoro polyether chain, R¹,R², R³ represent hydrogen or a hydrocarbon residue and R⁴ is ahydrocarbon residue having six or more carbon atoms, and ##STR7##wherein Rf is a perfluoro polyether chain, R¹, R² and R³ are eachhydrogen or a hydrocarbon residue and R⁴ is a hydrocarbon residue havingsix or more carbon atoms.
 2. A magnetic recording medium as defined inclaim 1, wherein said base film is selected from the group consisting ofpolyesters, aromatic polyamides and polyimides.
 3. A magnetic recordingmedium as defined in claim 1, wherein the magnetic metal film layer is amagnetic metal material selected from the group consisting of Fe, Co,Ni, Fe--Co, Co--Ni, Fe--Co--Ni, Fe--Co--Cr, Co--Ni--Cr andFe--Co--Ni--Cr.
 4. A magnetic recording medium as defined in claim 1,wherein said protective layer is a film selected from the groupconsisting of carbon, SiO₂, Si₃ N₄, SiN, BN, ZnO₂, Al₂ O₃, MoS₂ and SiC.5. A magnetic recording medium as defined in claim 1, wherein in saidperfluoro polyether derivative R⁴ is a hydrocarbon residue having 10 ormore carbon atoms.
 6. A magnetic recording medium as defined in claim 1,wherein in said perfluoro polyether derivative R⁴ is C₁₈ H₃₇.
 7. Amagnetic recording medium as defined in claim 1, wherein said magneticmetal thin film layer is a single layer.
 8. A magnetic recording mediumas defined in claim 1, wherein said magnetic metallic thin film layer isof a multi-layer construction including at least two layers of stackedmagnetic thin film having a non-metallic intermediate layer interposedtherebetween.
 9. A magnetic recording medium as defined in claim 1,wherein in the perfluoro polyether derivative, each polar group moietyrepresented by the formula ##STR8## has a molecular weight of less thanor equal to
 20. 10. A magnetic recording medium as defined in claim 8,wherein said non-metallic intermediate layer is an oxide of a metalselected from the group consisting of Cr, Si, Al, Mn, Bi, Ti, Sn, Pb,In, Zn, Cu and mixtures of any of the foregoing.
 11. A method for makinga magnetic recording medium comprising:providing a base film havingfirst and second opposed surfaces, said first surface having acenterline mean roughness, R_(a), of greater than or equal to 0.0015 μmand less than or equal to 0.0070 μm and having a maximum protrusionheight R_(max) of greater than or equal to 0.0015 μm and less than orequal to 0.070 μm; forming a magnetic metal thin film on the secondsurface of the base film in a vacuum chamber by a vacuum depositionmethod to define a magnetic metal film layer on said base film having anexposed surface; oxidizing the exposed surface of the magnetic metalfilm layer to define an oxide layer thereon; etching said oxide layer toremove at least a portion of the thickness of the oxide layer by passingthe base film between a pair of electrodes delivering a power density ofat least about 1.6 kW/m² and bombarding the oxide layer with an inertgas to provide an etched oxide layer having a thickness of from about 20to about 230 Å; and thereafter, forming a protective layer having athickness of from about 20 to 230 Å on said etched oxide layer withinthe same vacuum chamber, the combined total thickness of said etchedoxide layers and said protective layers being from about 40 to about 250Å.
 12. A method as defined in claim 11, wherein the inert gas is Argongas.
 13. A method as defined in claim 11, wherein etching is performedby bombardment with a mixture of an inert gas and a reducing gasintroduced into the inert gas, said reducing gas being selected from thegroup consisting of hydrogen and acetylene.